Any BMW with less then 100,000 miles and below $5,000 is probably going to be a good deal.
www.kbb.com (kelly blue book) is a great resource for pricing a car. Reliability wise, the 318ti was alright. You shouldn't expect Honda/Toyota like reliability, but it should be better then most cars. Personally, I would go after a 325i over a 318ti because BMW has much more experience building 6 cylinder engines then 4s, the extra room in the trunk, and the extra power is very noticable. I would also bring the car by a BMW shop to check it over.