Largest yacht for experienced owner/operator?

Before they moved strictly to chartering, I think my parents' experience might be applicable here. They were in their mid to late 60's and 70's when they cruised around Florida and the Bahamas on a 50ft express yacht with just the two of them as crew. A few thoughts based on my observations of their experience, and overall experience on the water in general:

• Right off the bat, I think 70ft is pushing it for just two, non-professional, people who are new to the activity. Probably doable for an experienced couple, but it's not where I would start. Especially if as you say, you don't have much experience on the water. I'm not sure I'd even start at 50ft. My folks basically grew up on the water. Their parents were boaters before them, and over the course of their marriage they have owned all kinds of boats from center consoles, to ski boats, to go-fasts, to the express. Experience over many years is very valuable, and still won't always prepare you for what you might encounter out on the water. Being on the sea is very unpredictable.
• You're girlfriend needs to be completely comfortable scrambling all over the boat, moving quickly, efficiently, and SAFELY around all the decks for line and fender work. And I would not trust a Skyhook feature to save you or allow you to multi-task on something that big when you are trying to moor. You need to be attentive to the controls because wind and current can affect your boat in ways you don't expect, and other boats always seem to show up at the wrong time.
• Also, deck hand is a position where life and limb are at stake, and the stakes only get higher the bigger the boat. Your girlfriend, or whomever you put out there, are taking a risk when they take on that role. There is no time to be grossed out by a gooey line. She has a critical job to do and needs to do it with total conviction.
• In general, $#!& can go wrong in a hurry when you are out on the water. I've had the anchor come loose and spool out almost crashing my windshield, my dad fell overboard once with no life jacket on when their dingy came loose in 6-8 ft seas (fortunately my mom saw it happen and was able to run from the wheel and throw him a line...but she had to leave the wheel). My parents were on another person's 90 footer when steering went out and the captain fell overboard going back to check...and no one saw it happen. I've crested a wave only to see a half sunken shipping container in the next trough while I was cruising along at plane and had to take evasive action.

The point of all that is just to say that no cruise out is routine and the variables of things that can go wrong only increase with the size and complexity of the boat. I much prefer taking my 30 center console out because I just drop the motors and go, and if someone gets their leg caught between the dock and the rub rail, they probably aren't going to lose it. My $0.02: If you are dead set on doing this with a motor yacht, would start smaller for a couple seasons...look at something 40 ft range, see how you and your friends/girlfriend handle things and get some experience, and then look to move up in class.

Solo depends on your experience and common sense. Many yacht owners don't have either. I've had private big boats all my life. Along with commercial and military experience. The secret to solo docking is being prepared with your lines out and plan your approach. And go slow. And cruising, keep your run times to your ability to concentrate and run a proper watch. Stay inside as much as possible. Know your boat and its systems.
I have an 83' boat now and most often solo. I like the quiet without anyone else aboard. When cruising I only dock at fuel docks, otherwise anchor. I carry an I/O for shore runs. If I have a unique docking coming, I'll either have an experienced friend aboard or hire an experienced deck hand. I have a private dock where I winter and have a shop. In my youth I worked in boat and shipyards and owned a yard. I do my own maintenance, sometime hiring a helper or two.
You can setup remote control for the wheel and engines that let you control the boat from anywhere. And there are thrusters. I don't have either.

I agree with Capt J and others who are full time Captains. I have owned boats for 30 years. One sport cruiser 34 ft then a 50Ft aft cabin. Still... as I just purchased a 76 ft and I am VERY hands on with my yachts, the systems, and handling capabilities, a boat this size is a no brainer for me to want to have an experienced captain for some amount of time on board to learn the boat and have 100% confidence. These are big boats, and certainly a lot more systems to learn.

One question is do you just want to be adequate or excellent. Haven't we all now seen the AT&T "OK" series of commercials. Surgeon is "Ok." You clearly don't want to trust your life to him but you want an excellent surgeon. We could have been ok from day one. We knew the basics. Why not just buy boats and go do it. The answer was we didn't want to just be adequate. We intended so spend most of our time on the water and wanted to be as good as we could be. This meant a captain and lots of training on the water plus it meant school and that led to license and upgrade after upgrade.

The other question is simply how much work you want to do.