any air conditioning problems with 1999 sl 500?

AC problems are almost impossible to diagnose without doing tests on it. One of the best methods is to recharge the system with UV dye in the system. Run it for a while and then use a UV light to find the leak. Most shops will do this for you if they are used to doing AC work.

I'd have the seller do the diagnostic if possible. I've got a sl500 a couple of years older, with same problem of no cold air. Leak in the evaporator, which is behind the dashboard - about a $2,000 job.

Low coolant is always a sign of leak, the system is sealed, refilling the coolant is a short term solution usually lasts about 2 years the alternative(s) is expensive.

My '89 has this problem now (the AC used to work great until a week ago). The coolant (R12 Freon) has leaked out. I had a radiator/AC specialist repair shop diagnose it at the recommendation of my local MB mechanic. The problem seems to be blown hose. Cost to replace hose is $400 due to having to remove the air filter unit to get at the AC manifold. Then he will need to refill the lines with coolant. R12 is the coolant used in cars this old and since there are not so many on the road it is expensive: $300 for the coolant alone. My car is old but only 150,000 miles and otherwise running great, so I am taking the plunge on this repair. Just posting it because it is not as expensive a fix as some have suggested above.