Hi and welcome to the forum Jason!
You will find a lot of friendly helpful people here!
Specs vary accross all engines but if you're looking to maybe sell on it may be wise to catch one with all the toys: satnav, top hifi system, cruise, heated seats, etcetc.
I bought a z4 recently and would advise that investing in a bmw-specific OBD scanner that you could use when going to view/inspect cars.
'Carly' app and wireless dongle work great but there are others and a quick search of the threads here will enlighten you to whats available.
Check for oil leaks at the back of the engine cam cover, oil filter housing and below sump mating. check for split air intake boot between MAF and throttle body as well as any signs of overheating. full service history preferably as well as all the other usual checks but an obd scan with the ok of a willing owner would be highly beneficial too.
u wouldnt consider a 3.0i over the 2.5. More power, as good fuel economy with 6th gear and potentially higher resale value....
good luck!