Ooo! Lucky turtle!
Well, the next biggest investment besides the tank is the filter. For a 90-100 gal, you'll want something big. I use a Fluval FX5 for my 90 gal tank, and I love it. I haven't heard anyone who's made the investment of the FX5 who's not happy with it.
Now, these are ridiculously priced in stores ($300+), but if you go on ebay, you can pick one up brand new for around $200 with shipping, or sometimes even cheaper!
There are other options for filters. The Rena XP4 would do nicely, but I haven't heard too much about it. You could also go for two smaller filters. With your 50 gal, you probably already have a decent filter, maybe even a canister filter? Depending on the GPH of your existing filter, you might be able to get away with a waterfall filter, or a "small" canister filter.
In my 90 gal, I use a 300W heater (Tetra brand, I think). That keeps the temp just fine. In fact, I just took it out because it's getting too hot for the summer.
You can use this calculator
here to see the approx heater wattage you'll want. You can "add up" wattage as well, so 2 150 W heaters = 1 300 W heater, then you can put one at each end of the tank so the heat is dispersed better.
You may have to tweak your current lighting system so it fits the new tank, but that hopefully won't be too big of a deal.
Also, you may want to build an underwater resting platform. A 90-100 gal is a lot taller than a 50 gal, so you have have a small turtle, he'd appreciate it, I'm sure. Also, they love to use these to sleep on, since it's not too far to the top to get a breath.
I'm not sure if I've missed anything, but I'm sure someone will come along behind me to add it
