Here's a link to an Autotrader.com national search of 166 325i's for sale. Prices range from $2,495 to $17,900, (average price is $8,585) based on mileage/condition. It's very educational.
Otherwise, the following may help:
Ask them for their maintenance records.What has been fixed, how often has the oil been changed, has the maint. been done by the dealer? Has it been done regularly?
The dealer can prolly look up the VIN and give you a list of the repairs/maintenance that have been done.
Is the mechanic working on it now their regular mechanic? or do they have different non-bmw type mechanics work on it? You can always ask the mechanic if it's been worked on before and what was done.
If they have maint. records, check how much they've been spending.
Is there an owner's manual? If so, check the maint. schedule and call a local dealer to ask them how much various procedures cost. When will the next major maint. procedure be required and how much will it cost? How much does a brake job cost? a wheel alignment? radiator flush? regular required maint?
What condition are the tires in? How much tread is left? Does it drive straight? When was the last brake job?, wheel alignment? other repairs? what was done and how much did it cost? etc., etc.
You should definitely have an independent mechanic/dealer/Automobile club check it out. They more items checked, the better, especially the compression. I know in my area the AAA will provide a report of all things checked and items to be repaired/concerned about.
When mechanical things go wrong it can get expensive. But that applies to pretty much every other car these days.
Bottom line is that there is pretty much nothing like driving a BMW. For the money, it's the best you'll get. Even my 1983 320is, which I drove until 1996, was a blast to drive every day of the 13 years I owned it.