Congrats on the first step towards your new HT! Just positively deciding to do it is the biggest hurdle. I'm no expert, but I'll just throw some stuff out there. Not sure what your budget is but for what you're looking at doing, this won't be cheap.
With your current AC maxxed out, you'll either need to upgrade to a much bigger unit and run the appropriate feed/return ducts OR install a completely new and separate system for "HT/Office Only". I would think the latter would be the easier and possibly cheaper option. If your upstairs ws just one big room you might be able to get away with a large, split unit. Compressor goes outside, coils and blower inside in a free-standing unit. They are very popular in Europe and starting to make their way here. Or even a big mutha, 25K BTU window unit. But since you have a room within a room, you're going to need dedicated ductwork.
You're going to need a separate sub panel run upstairs. 100-amp panel comes to mind. The good thing about all this is that it's basically new construction. Nothing to rip out, no existing plumbing/ducting/wiring to work with or around. A clean slate, as it were.
Your situation is a lot more complicated then say, turning a finished, spare bedroom that already has AC and power outlets into a HT.
Don't know how handy you are, but don't forget all this has to be done to code and inspected, less the resale value of your home go to pot. Not to mention possible "We're not covering you" from the insurance company should something go wrong. I.E. an unfortunate electrical fire starts and when they investigate they find out it was in the wiring you did and wasn't inspected. That's bad. I had a recent experience w/this. I could not install a single 240v outlet in my garage for a tablesaw b/c the insurance company wanted a receipt from a certified electrician that it was done to code.
And the outlet is exactly 3 feet from the main panel on the same wall.
Not trying to discourage you, but helping you think (I hope).