I have a 92 e30 cabrio (318i) and I love it, The maintenance has been cheap and there are a lot of enthusiasts that post detailed diy repairs and will answer any questions you might have. It seems that the maintenance is only expensive if you take it to a shop for everything. The 6 cylinders are nice for the added power but the 318s have the better gas mileage if you just plan on cruising around.

I bought mine with the understanding that there was a lot that needed to be done but when I started getting down to fixing things I found it was really easy to work on and the only thing that has cost over 10 dollars was replacing the tires which has nothing to do with the actual car.
older bmw's are the way to go if you want cheap maintenance imo