Most of these cars were cream puffs to begin with when traded, and considering the income of the average owner who purchased one when new, probably well maintained. Factor in the low mileage, and the first owner would have gotten anything major repaired under warranty.
Although I can't speak for all R129 owners, I have found the R129 to be relatively trouble free. No engine related problems, and major repairs so far involved a couple of dash light bulbs, a fuel line hose clamp, and a broken armrest slider switch. That's probably less than $20 worth of parts.
The most complex part of the car is the hardtop/softtop operation, and the threads that have surfaced in the SL forum indicate that the front latches may cause problems. The part is costly, but it is a DIY process.
I have found the SL to be a breeze when it comes to gaining access to areas for repair, but if anything larger than a fuse goes bad, expect the part to be mucho $$$!
Case in point. In the process of removing the dash cluster to gain access to the bulbs, I cracked the clear lens. Trip to the dealer left me $327 lighter!!!
I suspect when more of these cars get long on the tooth (100K or more), we may start to see those horror-story threads about $3000 MAS this or $4000 ABS that! But for now, it seems like the R129s are pretty trouble-free.