I agree with your looking for a 240D; they are, indeed, quite reliable and robust.
Edge offers you some good advice--don't buy the first one you see. The price of $4500 for one with 16X,XXX miles also seems high to me. but condition is everything. Just for reference, about 5 months ago, a 240D with only 35,XXX miles sold near me for a little over $5000. It was "like new" inside and out. I bought mine in April with 239,00 miles for $1000. It was in decent shape, but I am finding more rust as I have time to more thoroughly examine it. Rust not only affects the body, but all the pieces attatched to it. I had to replace all the metal brake lines, and all the brake cables.
A few items to check very closely--the fit of all body panels, operation of the vacuum door lock system, does the A/C blow cold? ( if A/C is important to you check this carefully as these cars can be expensive to repair). Using the information on this forum, check all the likely places for rust, ( floors, jack points, trunk floor, cowl). I believe one member has posted a fairly comprehensive pre-purchase inspection checklist--use the "search" function.

In spite of these caveats--I still reccommend these cars. I've put 6,000 miles on mine since April. About half of those miles are for business and I get reimbursed at 34.5 cents per mile--the car is paying for itself!
Still, I wish it had just a little more power....( I say the same thing about every car I drive!--but its really true about the 240d)

Michael