Q: Where could I buy a twin turbo kit for a 1988 BMW 325is for a reasonable price (2-4k)?
A: You are already getting a lot of power per litre from that engine... a turbo adds a massive hot-spot under the bonnet, and heat can damage so many things in that environment, you have to be very careful with placing a turbo. You also have very little space under that bonnet to fit a turbo! The engine will need stripping and the pistons machined to drop the static compression ratio, which is expensive and time-consuming. Turbos only give power at higher revs really, whereas a supercharger gives a lovely surge of torque through the rev range. If a turbo blows, the engine is in trouble... If a supercharger belt fails, you still have the engine. See if you can fit an Eaton supercharger, one which Mercedes use on their cars of similar engine size. Almost anything is possible with time money and expertise, but you can't do this on the cheap, or it will bite you in the a$$.