Spinedoc, I have a '90 300SL with 5-speed manual tranny. Because I think I may prefer an automatic at this point in time, I have driven several 300s and 500s for sale with automatic gearboxes and have made comparisons.
The power of the 300s with automatic is entirely adequate. The Star
magazine from June/July '89 gives an estimated 0-60 time of 8.4 seconds for this model. Not bad at all. I do feel that the automatics are a little sluggish off the line compared to my car, however. And hey. The straight-6 is just incredibly smooth.
Driving the 500s left me slightly disappointed in the acceleration. The difference between them and my 300 simply isn't as dramatic as I thought. Obviously my expectations for the 500s were a bit too high. Still, with a full-throttle induced kickdown the 500's performance is truly exhilirating. If I decide to buy an automatic, I will definitely spend the extra bucks and get a 500.
Finally, you mention a budget of $20K. Not sure about the market for these autos in the northeastern U.S., but in California a pretty decent '91 500SL can be had for less than that. To wit, this car sold outside of eBay for just $19K: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=1868153869.
I'm in Vegas and plan to scoot across the border to do my shopping when I am ready to buy.