Have money set aside to repair it. Owning a modified car requires patience, money, and a good relationship with a local performance shop. You WILL break things, and you will always be in jeopardy of blowing something expensive up.
depends how modded you are buying. I bought a HEAVILY modded forester 1.5 years ago. Depending on how modded you buy you may have to take out a personal loan or add you own cash on top due to not being able to finance it all through a bank.
A simple stage 2 modded car with some light suspension mods shouldn't fetch too much over market value.
If it has engine mods see that it was properly tuned. I'd want to see a list of exactly whats actually on the car, and if certain parts were removed. If it was tuned with an AP make sure you get it and with all the maps, or be ready to get it retuned for whatever assortment of parts are actually on the car. The rest would just be stuff you would look for on any car; worn wheel bearings, clutch, transmission/diff noises, signs of headgasket issues, leaky/torn cv boots, brakes/brake fluid (i've seen a lot of cars for sale where the brake fluid has never been changes), worn bushings and odd vibrations. I wouldn't hesitate to buy a stage 2 car but I'd probably skip ones with swapped turbos unless you're looking to start a project.