Ohtooman, unless it's a euro car it most likely has SLS.
kashbaugh,
How much are they asking for this vehicle?
You should check for the following:
1) Rust under doors and in the floor boards (pull up carpet and look underneath car for rusted out panels)
2) Unusual driveline vibrations under acceleration and at speed (should be rock steady and no buzzing, vibrations or unusual noises)
3) Look for evidence of leaks at the sunroof (if leaking it may be nothing more than a clogged drain but can be much worse- see this:
http://www.peachparts.com/shopforum/showthread.php?threadid=68114)
4) If you buy it, make sure you get the roof rack cross pieces (replacment cost $200+)
5) Smooth shifting, no flaring or blown shifts
6) No delay shifting into reverse
7) Check for excessive blow-by (do search on this forum for more info on blow-by)
8) Check self-leveling suspension. Do it this way: Find 3-4 'plus-sized' friends. Open the rear hatch, have friends all sit in the rear cargo area. Measure distance from top of rear bumper to ground. Start engine, let run. Now, measure distance from top of rear bumper to ground. Distance should be greater than it was before you started the engine.
9) Inspect rear suspension (shocks and accumulators) components for leaking hydraulic fluid. If there are any leaks come back to this forum and search on "TD Suspension, hydraulic suspension or SLS" or PM me and we'll go from there.
Obviously, there are many other things that you should take into consideration. What I've listed here is the significant things I could quickly think of.
IMHO, the W123 chassis TD has to be one the finest road cars ever built. Mine's absolutely quirky as hell but I love her dearly.
Good luck,
R Leo