Check vin numbers for recall on drivers air bag replacement; only some where effected.
Open door and look under door at rubber seal for rust.
Take it for a drive; wing up at 65, down at 45. Use the s/w switch on the automatic and run through the gears manually.
Check to see if anti theft alarm/siren functions when red patch on key FOB is pushed; applies only on North American sold cars.
If you have a diagnostic scanner plug it in and scan the car as a Mercedes 2004 slk 320 (it is an AMG slk 32 if supercharged) Stored codes? active codes?
There is a large plastic pan under the engine. Is it clean?
Run the A/C and heater.
If it is a roadster has the rear window been replaced or the entire top? Window is a know issue.
I would check the area under the rear trunk for water. There are several methods; check it out via the search box upper right on the forum.
Ask if the sticky key fix has been completed and search for it using the box upper right. RCM soldered or replaced? Crankshaft Position Sensor original? Search CPS.
These cars run well and the engine is very durable, but you have to be willing to do most of the work yourself, as they are an orphan child to both Chrysler and Mercedes; Best bet if you must is to use an independent garage that specializes in Mercedes.