325e are a 2.7liter 3500/4000 rev imit
325i/325is are a 2.5liter 7000 rev limit.

The i motors are the sought after because they rev.
people combine the two. they use the 2.7 bottom end with the i cylinder head.
combine the dispalcement of the e motor(bottom end) with rev characteristic of the i motor(cylinder head)

There's alot of info you'll have to search.
For the turbo, you'll be well off buying the book Maximum boost by Corky bell
Everthing you need to know is in there.
It helps you understand everything there is to know as well as designing your own turbo system.
e30tech.com also has a ton of info on turboing the m20