Great choice in the new motor BTW i loved my E46 M3, you will need to have deep pockets for the M modding game though.
the best MODs i done to the M3 were Eisenmann Race exhaust and i can recommend the HSD coilovers Brilliant, Upgrading your brakes should be top of the list though (if you like a track day or two)
a liitle list of things to watch out for.
Rough Diff (can some times be fixed with an updated oil) although many have required a new diff 2.5K
Seized caliper ( Very Common )
Watch out for cracked subframes ( reasonably Common and a 4-8K Fix )
Rust under the door catch cover.
Trailing arm bush ware, If fecked i would recommend upgrading to OEM CSL bushes With rouge engineering Bush limiters, this keeps OEM comfort with Poly ridgidity.
You are gong to love It mate, best car ever. I just found it to expensive to get it the way i wanted it, it easily could of absorbed 10k, roll in the GTI