Your Original post is asking for advise and opinions,
alot of people have already suggested buying a alternative Z4 but it seems you have made you mind up before posting this thread and are not prepaired to listen to anyone unless its along the lines of
"Go for it" re; Engine swap.
Did you read my
"Inventing something that already exists" comment or is its not what you want to hear? Your re-inventing the wheel (re-inventing the 3.0lt Z4, which has been done by BMW 10+ years back)?
Its not that i have anything against you or the end product in question, but i will NOT be "blown away" or impressed by anyone that chooses to turn their Pre-facelift 2.0lt E85 into a 3.0lt one. Why anyone would is beyond me, when if you look through PistonHeads Classified you could find 100 3.0lt Z4's for sale? Also some of these cars possibly alot newer and superseed your current car (not just Sports seats, multifuntion wheel etc... the base and internals), with newer parts which would be the best possible base for any further engine mods.
For the record (based on that i have previously "Changed"* many engines in cars) i say: Buy a Silver 3.0lt Z4 E85, Swap all your Mods (Wheels, lights seats etc..), THEN go to work modifying that car.
Also maybe stop being rude and having ago at people that give you advice with your repeated on going theme that you dont agree with.
We all love our own Z4's, but i wouldnt hesitate to buy a different more powerful car if thats what i wanted (which is what i am currently doing)
* "Changed" to bespoke engines, not something that could easily be bought