I tried eight 500s before buying a 1997. Its closest rival was a 2000, but the 1997 was in better overall condition. The differences between earlier and later models were difficult to detect, but I came away with a slight feeling that there was no performance difference to worry about in everyday driving and that the earlier cars felt a bit more solid. I helped a friend buy one, and the one thing he regrets about his 1994 car is that the later interior door panels are tidier and less intrusive (speaker configuration).
As far as the wheel size is concerned, a previous owner changed mine from 16" 8-holes to 17" windmills (Albali). Initially I wished he hadn't done it, but over the four months of owning the car I can't really complain about the ride and I have grown to like the appearance of the wheels.
I archived the following, so you may find it elsewhere in this forum:
I've just been through this process. Search Autotrader and Pistonheads for starters. There are plenty of R129s around, with the 500 in a minority and the 600 in a tiny minority.
My impression of Cheshire Classic Benz is favourable, but that's an wallet-wincing price for an early car, even with moderate mileage (check CCB's own site for the downside of a low-mileage car). There are dozens of cars for sale in the £5000 to £12000 range, and a few at higher prices. You don't say where you are, but it seems to me that prices are often less realistic away from the South East; that may seem counter-intuitive, but I suspect it has to do with local availablity of cars and the intensity of local competition.
Buying from a dealer does make sense in many circumstances, but from my limited experience the supposed benefits are elusive. Wherever you buy, budget £200 or so for a pre-purchase inspection by someone who knows the model intimately. Walk away from anyone who resists a PPI. Look out for traders (no garage/showroom) wanting to be seen as dealers; this is not so say beware of them, but they offer little if anything beyond what you'd get from a private vendor. I can hardly say "beware": I bought my car from a trader, but I reckoned he was trustworthy and we established early on that he could offer little over a private vendor.
The most common fault I found among those I test-drove was faulty or failed A/C. Corrosion is rare, but it does exist. Watch out for extensive bonnet rash too, touched in or not. Above all, make sure the soft-top works properly. I don't like the special editions (interiors, mostly). Some are not so special: I inspected and drove one that had no memory function on the electric seats -- the worst option of all (as I know from my E320 Coupé). It also had 18" wheels (275/35 rear and 245/? front), so a bad ride and so much tramlining that I was glad to get back in my 1984 Capri 2.8i.
There's also stacks of stuff on the Web. Try these:
Info about the R129 SL class | BrianClifton.com
Mercedes SL500 R129 1990s History
And there's an active R129 forum at:
Benzworld.org - Mercedes-Benz Discussion Forum
Let us know how you get on.
. Start with this one 
