It comes down to this.
BMW aren't keen power people when it comes to Super Chargers and Turbo systems. They haven't really had much to do with them in the past even though they have played with them.
Remember as well, that a Turbo engine and an NA engine have a few differences. It's not a simple matter of plugging a couple of turbo's on and away you go.
To turbo an engine (in this case) you'll need to lower the compression ratio to a modest level of at least 9, or probably the usual 8.5. This can be achieved by adding a new head gasket, however, you'll find that the internals aren't really setup for it, so you'll also be looking at new pistons (probably forged), new conrods, and bearings and seals. By now after this, getting all these internals and changes done will be up around the 6-7 for just the mods. We haven't even added the turbo/SC yet.
Now add onto the fact that you'll need to upgrade the clutch, and that'll probably have to be a custom jig because so few people would really be looking for much more performance out of these cars, unless they're universal clutches in BMW's (I doubt it). That's upto another 2K there (more likely around 1500).
Now that you've done these you'll need to put your turbo/sc on. The bonus of a SC is that you won't need to play with your exaust manifold, but for more power a SC will/might cost you more.
Eitherway a system like either will cost you minimum 2500 for a good job.
You'd be lucky if you got 3500 though for that sort of thing.
Then you'll need to upgrade your injectors to deal with it... Don't know the cost of these unfortunately but expect a bit more, as well as your fuel pump likely.
After all this, you'd be pretty lucky for the standard ECU to be running nicely, so you'll need a piggy back system or something of that kind to keep it running.
Once you've done this... Well, yeah, you're looking at big costs anyway, and it doesn't add there.
You'll at least want to think about upgrading the brakes due to the new weight of all the parts on front and suspension too probably.
You'll need to upgrade rims with the new brakes to they fit as well. All up you're looking at at least 4000 for that, unless you get something like other manufacturer brakes, 2nd hand brakes etc.
And btw. These are only some of the things that you'll need to worry about. It's just the way it is. Then it's a matter of keeping the car road legal at the same time.
The problem is that when doing a turbo upgrade on one of these cars you could probably get another 50-60HP maybe up to 100hp out of them without too much risk standard by bolting on, but that's limitted to that until you upgrade the engine. Otherwise you run the risk of wearing it out way before it's time.
And the gear box... Chances are you'll need to custom jig that up to your car.
My personal recommendation is to attach a Getrag gearbox, but once you do that, your Diff will need changing too, or you could end up having a top speed of 150 and dodgy gear ratios.
Oh yeah, a second hand getrag will set you back at least 3500. A brand new one will set you back at least 7000.
There's the matter of the clutch as well by the way, if it be a push or a pull type. ^_~
Not worth it in the end. You want a quick car for the price of that BMW? Japanese car is probably the easiest way to go.