hi bro, personally would recommend a 3.0 but 2.5 is a sweet engine non the less. hp is abt 193 (same engine as e46 325i). maintenance is about the same as an e46. but bodyparts cost slightly more though since their numbers are small (i get most of my parts from ebay). before agreeing for a purchase, make sure u bring along a reliable mechanic or send it to a bmw specialist for inspection. they may be able to reduce the milage on the meter but diagnostics will detect it. make sure all electrical functions work..especially the softtop. font worry to much abt japan spec because the radio is a world radio..but satnav navigation cant be used though (models with satnav screen). if uk spec, check for rusts (because they spray salt on the roads during winter). japan spec usually hv lower milage n better condition.
i've owned my Z4 almost 2 years now and no problems what so ever (touch wood). it is a really wonderful drive.
p.s contemplated on getting elise, boxster, slk before the Z4..but the Z is the best all rounder.
cheers!