What problems should you look for? Well when i mean i wouldn't buy one, i didn't mean I (myself) wouldn't buy one, i was just putting myself into the position of the buyer and saving them a headache. Its good you haven't had any problems yet but most of the problems on the 04 are minor and are just annoying. Such as:
MMI glitches which need re-flashing, MMI screen motors that fail all the ...