gilnsammy,
If you're willing to at least tinker a bit a 123 is ok. If it's a 'take it to the shop' thing it could be expensive depending there on if you have a good shop in your area. The MB dealer will rape you...that's is if they don't get all snooty and just refuse to work on that old thing. I too like the CD's but I wouldn't dream of owning one if I had to go to a shop to get it repaired. If you must have one get a pristine one(mechanically and "with records"). CD's are more expensive than the other 123's but if you get a good one to start with it could be a good relationship. If not it could be a serious money pit but your local garage will for sure send you calendars and Christmas cards every year forever...prolly from HI. while he's there vacationing on your money.
So far as the gaso-vs-dsl thing I have no idea but I lean toward the dsls for reliability. Good luck.
PS..it's a good car to learn wrenching on and it's never too late unless you just don't like that sort of thing but if it's not your only car it's sorta fun in a weird perverted sort of way.