For the years you're specifying ('87-'94) there are two body styles. The 107 body was sold in the US only as the 560SL '87-'89 (its last year). Unlike earlier 107s, it's got a driver-side airbag and particularly for '89 a few suspension tweaks. It's built like a tank ( it was known to DB engineers as der Panzerwagen--the armored car), and if well maintained the 5.6-liter V8 is pretty bulletproof. Downside-Be careful of the specific car and get it well inspected by a knowledgeable MB mechanic, though that actually applies to all MB. Fairly thirsty-14mpg or so in town, up to 20 highway, on premium (required). The basic design was done in the late 60's.
1990 saw the 129 body. For your daughter, stick to the 300/320; less expensive to buy, probably a bit less to run, and still enough power to get into trouble! The 500 is overkill. The basic design is about 20 years newer than the 107, mileage should be definitely better, and although the 107 is quite sturdy the 129 is inevitably a safer car due to much more recent design. Unlike the 107, it does have an electric top--something more to go wrong but nice to have. A really well kept late 560SL probably won't cost much less than a good early SL300. I'm not aware of any particular problems with any of these engines.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)