Hi Tom,
These are GREAT cars - not that I'm biased at all
I would highly recommend a pre-purchase inspection by a M-B specialist to assess the condition of the car and if it needs any major work. Also do a search here on "SDL" and read some of the information.
If your friend has owned it for some time and the car has a good service history then that is a great start. Regular maintenance is critical on any car that is 18 years old and vital for Diesels. Regular oil changes and all the filters is a good sign the car has been cared for.
Potential problems? Well apart from the obvious (Brakes, Suspension, etc.) the SDL uses the 603 series engine with an Aluminum Head. If Overheated they have been known to crack but otherwise appear to be just fine. Replacements are expensive however.
Transmissions are good if maintained but symptoms of possible problems are slippage between gear changes. They tend to be a little hard shifting but really harsh changes may also indicate a potential problem.
The Sepentine Belt that drives all the accessories (water pump, alternator, A/C Compressor & Power Steering) has a pulley Tensioner that is prone to failure. The Shock absorber on the tensioner usually goes first but the pulley and tensioner bearings quite often fail. You will hear a chatter from the front of the engine.
If your friend or previous owners haven't already changed the Vacuum Pump then this should be added to the preventative maintenance list. If it has a smooth front cover with no screws visible it is an original. They are normally fine but can fail without warning and in doing so can deposit their bearings into the engine
with obvious results. The newer design (with screws on front cover) have sealed bearings to prevent this. Replacements are about $350.
The Auxilliary water Pump which circulates coolant through the heater at low engine rpm's is another common failure item. It is a pain to replace due to its location between the bulkheads and is expensive. It can also damage the Climate Control system and the A/C Compressor Clutch if it fails as they are all on the same circuit. You can disconnect it though (I doubt you need it in AZ) and run without it.
Radiators are a common problem item as the plastic header and bottom tanks often fail where the main hoses connect.
$1500-$2000 is a good deal for a 180,000 mile car if it is in good condition with no major issues.
Good Luck