Definitely make sure the top mechanism is functioning properly. Do a search on the forum and you will find topics about hydrulic failure in some models.
Chech for oil leak as well. Some year model is more prone to have gasket failure prematurely. I will definitely be more careful on the SL600 since the engine is pack tight in the bay, and any minor problem will equate to pulling the engine. That means big bucks.
Same as radiator, now that many of these SL are getting up on years. You may want to double check to make sure the cooling system works fine, although you cannot rely on visual check.
Always look for one with full service record. Stay away from one with spotty record, they mean money pit, not unless you are a very good diyer. Other than that, the SL is a very good and reliable car. I just added up my last year maintainence cost for my SL, a merely $138.00 !!
04 Infiniti G35.5 BS