If you want to keep the car you have but want more power, then you shoudl get hold of a complete 325i or is engine......don't mess around with your 325e block and i heads, its not as simple as that!
Basically your engine is built to rev lower and deliver better ecconomy, so the head componants have less durabiltiy as they are not expecting to see higher compression or revving!
A straight engine change isn't so bad to do.....engine mounts are the same and so is the transmission bolting......buy an engine complete but you may have to check the wiring harness carefully......the 101 connector may well be different......so research the compatability of the engine harness on the 325e, hopefully you can change that onto the new engine and plug it in!
Consider a clutch change whilst you are there, you won't want to drop the gearbox too many times I assure you! In fact I will remove the engine and gearbox combined in the future to do any major work as its easier than getting to the bolts and starter when the engine is in place!!!
Have a go but keep it simple if you want to keep the cost down! As mentioned, any odd engine combination, can get expensive and not always reliable!