Heres what i can give from my experience.
After you find one that looks good (interior/exterior), look out for the following when doing a test drive:
-Steering should be quite stable (i test drove a few when hunting, at least 2 had some looseness and disconnected feel)
-When driving over humps, the suspension should be firm but well "cushion-ed". (slowly of course) I drove a few that gave a clunk when going up a hump; like the one i eventually settled with, the bushings in the support arms were worn and needed replacing.
-Dont ignore the error codes if theres any, some may be a simple fix, but others need a whole lot of $$ and time back at PML (and PML only) to fix.
-Open the doors and leave them open. They should feel firm when you pull or push them. If they close on their own when left open, you'll need to replace the door brakes. Its a frequent issue so you'll have to live with it.
-Mine had a serious oil leak that killed the alternator and battery. Not sure if the 730s have the same issue though. (Mines a 750) Still, its worth a look around the engine bay just to make sure.
You're right about maintenance, it could cost a little more too. But its not so severe that you'll need frequent overhauls or leaving it at the workshop for days. Its a reliable car
Im sure you'll enjoy it as much as i do too! HTH!