After buying my 325ci, about 2 months ago, definitely one to consider! Absolutely lovely machines, cracking engines.
As for things to check, I'd ask about whether or not the water pump has been changed, at that mileage it really should have been. Mine hasn't had it done just yet, but it's on my to do list. Also listen out for knocking from the rear of the car, could point to sub frame issues, or cracked springs. They are pretty reliable cars all in all, I did a lot of research before going for one as people on this forum will know!
As for the fuel consumption, going purely by the on board comp, around town it claims I get 26mpg, but in reality it is most likely a lot worse than that. I don't really want to know what it really is tbh! It wasn't made to be an eco box!
If you're looking for more of a performance car, you probably wont find a nicer 6 MPS than this one:
Having seen it in the flesh on numerous occasions, it is absolutely mint. And also goes like stink!