What does AC not working mean -- it's not cold or is the whole system shot?
How much history is documented in records?
Any indication that a valve job was done?
My rule of thumb is if you're not familiar with the car when you inspect it, whatever you spend getting it up to your standards right off the bat, figure it'll cost you that much more in surprises the first year.
The 300SEL is a fine car but the big benefit is that it's easy to work on it yourself. If you're going to be taking it to a shop, consider a V8 model that in many cases will cost you less to purchase because V8s are more commonly available. Fuel consumption is pretty much a wash despite what your intuition would suggest.