The intake manifold can stay the same. The exhaust manifold will have to change. With the US models the compression ratio is low enough stock to turbo them without change.
The cheapest way to lower the compression ratio for the 1.8L euro m10 (10:1) is to swap in a 2.0L crank and pistons. The combustion chamber of the 1.8L head is larger and your compression ratio will drop from the 2.0 liter's 9.3:1 to somewhere below 9:1. You could also run a 2.0L crank, rods & pistons from a 320/4 which would probably set you around 7.9:1 CR with the 1.8L head. I don't know much about the euro pistons, but I would guess that the 320/4 used the same flat top pistons that the early US 320i did.
a turbo can give you 500% more power and much more if you build the motor to handle it, size the turbo properly and know how to tune it.
I have a small, properly sized T25 turbo (for my application), 8:1 CR, 2.0L, 280 cam and I'm making about 250 hp at 14 lbs of boost. I designed, fabbed and installed the whole system and it took a TON of work, but it was worth it.
I know of other 1.8L turbo guys still running CIS that are making about 200 hp, and I know of an m10 turbo guy running megasquirt that is making over 400 hp. It all depends on how much you want to spend. For more info on turboing your e21, read this:
E21 Turbo DIY thread
and see my website:
http://jrcook320.e21legion.net/