Nice choice on the GTS-T, they're a great value. Honestly, the RB20 is pretty underrated in my opinion. The power comes on really smooth. Some of my friends had GTS-Ts in Japan, and with some minor mods, their cars were about as fast as my R32 GT-R at the time. While I was struggling with basic maintenance, they already had a well-sorted car and were bolting up new turbos and tuning because they just had a cheaper platform to work on.
With that said, basic maintenance should be your first priority when you get the car. I'd budget a few grand extra to refresh suspension, brakes, change all the fluids, replace hoses, filters, sensors, plugs, coils etc. Those minor things add up really quick, just remember it is a 25+ year old car. Bolting up bigger turbos straight away can be asking for trouble down the line if your basics aren't taken care of.
Personally, having driven cars with the RB20, I don't think there's a need to change it, unless you have money burning a hole in your pocket. The RB25s are great, but if it were my money, I'd just keep the RB20, get everything squared away, and mod.
As far as starter mods, you're on the right track, but I'd like to add that an exhaust goes a long way on these turbo cars. With that said, a Skyline of any sort for sale in Japan is hard to find without an aftermarket exhaust already installed lol