Well for E46 to get more power expect to pay some $ out beucase BMW pretty much gets as much as they can out of the engine w/o using forced induction. Exausts are easy, same with intakes. Body kits and rims are also a common thing to do on E46's. If your looking for some more power but for around the same price, check out some used E34 M3's. You can find them all over the place for $10-20k.
Also just a FYI E46 engiens dont have timming belts, they have chains. Water pumps are cake to do on these, same with thermostat. I wouldnt worry about the Vanos. I rarely see them being replaced at work, and dont let the vanos be the deciding factor on buying this car.
Please dont get the 240, my friend got one and its slow as hell. If its in the same price defentaly go for the E46.