Here are some links to various discussions that should help you get smart about this fast:
http://www.mbnz.org/forums/forums/thread-view.asp?tid=604019
http://www.mbnz.org/forums/forums/thread-view.asp?tid=603764
S-Class Fiasco: W220 vs W140
The short answer is, in my opinion, to go with a '97-'99 W140 420 or 500. I would not recommend the 600 since you mentioned the word "budget" in your posting, and if you find a 320 you like, make sure it is the long wheel base version or your will always be looking at every other W140 you see in envy. If you go before '96 you have to worry about the evaporator, which you don't want to have to even think about. Make sure that you pay the money to take the car you want to buy to a good Mercedes dealer and have them spend an hour going over the car to tell you what is wrong with it. It is only a $100, but you are a fool if you don't invest this much in making sure you know what you are getting. Make sure they check shocks, bushings, brake lines, etc. You should already have checked every single electronic switch and feature to make sure they all work as they should, or you know which ones don't.
Good luck. You will never own a finer car if you buy a W-140, and you will never regret it a single day.