Need help on choosing a boat??

Look for an aft cabin MY with a Cockpit. Hatteras made one with a cockpit somewhere in the 45-50' range I think Viking made one as well...... as well as many others......Keep in mind if you're going to do the great loop, you have to get your height under 19'2.

You may be describing the impossible dream. You can fish off anything, but as soon as you talk cockpit or lower station you're taking away living space. As soon as you talk performance you're taking away economy. Bridge brings clearance issues as well as a possible overhang above the cockpit if you head towards sport cruisers. Since you're talking about living aboard and cruising the loop you might want to consider towing your 20 footer for fishing, but that brings issues of its own. I think you'll need to prioritise and compromise to stay in that 45' range. Let's start with your definition of fishing. Are we talking about sitting on the hook catching dinner or trolling for marlin? Does performance mean 18 kts or do you need 30 kts? Does liveaboard mean you and your squeeze or that plus 2 teenagers?

More boat than you may want or need, but maybe it's worth looking at a Mikelson 59.

Mikelson 59 Long Range Cruising Sportfisher

First, Hatteras are terrific boats, and love the older ones, but I would stay away from the convertible. It's most likely been used for charter fishing which means rode hard and put away wet if you get my drift. The exception would be if someone already did a refit recently, but you'll pay for that. Your guest list is very ambitious for the size you're looking at. Remember, guests and fish start to stink after 3 days. With 2 teens you're looking at 3 staterooms or there will be war on the first rainy day, and the third SR on most boats under 55' is seldom more than a glorified closet. Most of the fishing you talk of can be done off anything until you get to shark & tuna. Then you're talking about a bit of speed and heavy water. You might find that it makes more sense to get a cruising boat, and charter for the day when you want to go for big game. In fog you'll probably prefer to be on the exposed bridge for better visibility and hearing, but I agree a lower station is almost a must up your way or else you end up with a very short season. Doing the Loop is something you'll probably want to do after getting a few years of experience. So the bottom line is that you may be needing to work a few more years and maybe marry off a kid or two before you go for your dream boat.
I'm going to suggest that you look seriously at that 32 GB or or maybe a 36GB. They're classics, and will maintain their value. Good place to get your feet wet. They'll serve you well as a weekend getaway or vacation boat, and they're also ideal for the Great Loop. The older ones are fairly slow boats, somewhere around 12 to 16kts, and handle well even in bad seas. They also tend to be owned by people who take good care of their boats, and are fairly economical to run. After you've cruised for a couple of years you'll probably want to move up, probably to something in the 50' plus range depending on you finances. By that time you'll also have enough experience to decide if you want to move to the coast and fish (SF) or cruise more inland waters (trawler, sedan or sport cruiser).
You and Capt. J are both right on the height. 15' will get you under all the bridges on the Erie Canal, 17' will get you under the east legs. This is the first time I've heard the 19'2" referred to, so I'm not exactly sure where he's thinking of but you can cruise most anywhere though at 19' with certain exceptions as mentioned, and you can usually arrange an option of reducing height by dropping a radar arch, but it's not something you'll want to do often. I think he might be referring to avoiding bridge openings on the ICW, but we'll wait for him to come back and educate us more on that.

With a purchase budget of around $200,000 the question is:
What is your annual maintenance budget?

Suggestions from NYCAP & Capt. J are good.
Yes Hatteras are terrific boats and if you can find one where the restoration and update is done, then you are on a good start......depending on whether you are prepared to accept the annual costs. Many posts here on these costs and how to calculate them. One major repair on a 42' Hatt or GB can be anything from $5K to $25K or more.
A very different type of boat ownership than running a 20' CC.

As for multi uses on a particular boat: I have caught Mahi Mahi, Marlin and Tuna off the small cockpit of a 1980 50' Ocean Alexander Mk1 trawler/pilothouse which is a full displacement hull running twin Ford Lehman 120 hp motors with a top speed of 10 knots, with wind & tide. So trips were well planned with an acceptance that if we hit bad weather, we could not outrun it as in a SF, but endure it. Had some bad trips, but never felt in danger as it was a good solid boat. There were offshore fishing trips that lasted 2 days longer than planned due to weather or choosing the safer conditions to come through an inlet and home. This was one of our favourite boats, but, it is all a matter of compromise. You cannot have it all.
I would suggest you look at a Hatteras 53 or 58 Yachtfish.
Just remember, with that budget, be prepared to own it for a LONG time and that you will be spending the same amount on making all the systems right.
Just my 2c.

A grand banks 42 or 46 would fit your wish list

I'm kinda' partial to my old 40ft. C-C woodie Sportfish.

19'2" is the max height for the loop. The lower height I think it's around 16' something if you take the Erie canal all of the way into lake Erie. Otherwise, on the Hudson river there is a bridge that's 20'2 north of Troy, NY, then you go west on the Erie Canal to 3 rivers, then North to Oswego, NY and into Lake Ontario, then west........the next lowest bridge is just South of Chicago between Chicago and the Mississippi which is 19'2. I did the entire Great Loop in 2008 on a 75' Hatteras MY, we got the height down to the FB windshield height of 18'6 and had plenty of room to spare......about a 1' at the time. You could also flood the bilges if you need another 4" on most boats.....but wouldn't recommend that unless you're very experienced.

What about something like this: 1984 Kong & Halvorsen (Island Gypsy) Motor Cruiser Sedan Power Boat For

How about a Huckins?

There's a few Huckins in your price range that check most of your wish list boxes... aft cockpit, can kick up a wake when wanted but are fun to run at hull speed, and good cruising layouts. Of course the wood/fiberglass hull is the catch... a survey by someone knowledgeable about that particular build method is a must...

I manage a 51' Cham Hwa which just changed hands and sold to a new owner. It had CAT 3208's, was fuel efficient but cruised at 14 knots, has a cockpit, was in good condition mechanically and interior, 2 large staterooms, lower helm, needed varnish and compounding and waxing on the exterior and the electronics. Yet sold for $84k. But all in all was a good boat that just needs a little cosmetic tlc. So they're out there......

Halvorsen are highly regarded in Australia.
The Havorsen family migrated from Norway to Australia many years ago and built hundreds of boats here before moving the construction to Asia. I think it was Hong Kong.They built some beautiful larger pilothouse boats as well, but they are hard to come by.
Here is the web site:
The Halvorsen Tradition, Since 1887 - a History

My wife and I finished the loop last year on our Ocean Alexander 54 pilothouse and the boat was great but not perfect. The size is very comfortable but dockage is hard to find in some areas and expensive. The bridge clearance of over 16ft. can also limit where or how you can go. We are 18.5 without folding down the arch but didn't go all the way through the Erie Canal because of it and had to bypass downtown Chicago this time around (clearance downtown is 17ft.) We also started up the coast through New Jersey and New York in late April and most places don't really open until the end of May. The trip down the Mississippi from St Louis can be a stinker but the rest of the trip is great. Good luck with your hunt for the perfect boat, I am on boat 32 and have not found it yet!

As said, there is no perfect boat out there but I own a 48 Californian that I plan on doing the loop with. She is 3 stateroom and 3 full heads with plenty of room. Although she does not have a cockpit (the 55 does) she is well suited for the task at hand. Even with DD 6-71 TI's, she can be miserly but has the potential to get up and go.

Chris