That allllll depends on what power you are looking for. You can get a peppy car outta it with just an intake/headers/exhaust/maybe some ECU work and tuning. Don't dare expect to break 200 bhp. If you're looking for more power, and a turbo-charged application, you can't go wrong with a USDM WRX engine. You start with 227 crank hp and you can start quite a lot of mods. People are selling them quite frequently
Now I'm not going to slam the turbo-an-RS-idea. You CAN run a a low-boost setting on the stock RS block for about the same money and have a safely done forced induction car. However, it's going to require a LOT more TLC, no matter what. And the best way to go would be to have a shop do some fine tuning work with you. But it CAN be done. Many people have done it and have blown blocks, so beware. The common consensus is running over 5-8 PSI is a big risk on the stock block.
If money isn't TOO big of an issue, definitely check out the swap idea. WRX engines can be had for decent prices. If you're really adventureous with money, check out a USDM STi engine. That would be quite the sick car. Remember that with an engine swap, if you don't know the in and out's of doing it, you'll be paying someone else to do it. Especially disconcerting is the wiring harness and the pain it can be. However, plenty plenty of people have done WRX swaps without a hassle as long as you have the know-how and help to do it.
Regardless of anyone who thinks turboin' an RS will yield a good car, there is NO debate that a WRX or STI swap would yield a MUCH more reliable car for modding.
It's also important to note that the WRX tranny is not the most beloved item from Subaru. They break easily if run hard. You'll want a more strengthend tranny if you do need some big power or if you abuse the car, haha
So again, if you just want a daily driver with good suspension characteristics, just go with intake/headers/exhaust/possibly ecu/maybe valves/camshafts, etc. You'll be very happy and it'll be fun, especially in corners.
If you need to push beyond 200 bhp, you're going to need an engine swap or a turbo application of some type.
Also realize that since you're checking out an '01 RS, you have the benefit of a lighter chasis than the newer 02 GD body. IIRC, it's about 100-150 lbs lighter? However, people have generally said that the GC8 was a lighter body, but that the new GD body had much better suspension characteristics. Doing an engine swap will yield quite the nimble beast in the older body.
Hehe, or if you're loaded with money, check out the guys at TWE and you'll make you a ridiculous naturally aspirated 300 hp beast. They just did it
Good luck man!
P.S.- I have an 02 RS, and after a year of it... to me, it just isn't enough. I'm not even a horsepower addict like some, but I definitely want a turboed car. So don't settle for the bottom line. If you need more power, go for it and put that money into something that'll satisfy that need!