As a 95SL owner, the high mileage would not be the factor for me when considering the purchase. In fact, the most troublesome SL examples are the ones that sit in the garage for years, allowing the seals to rot and fluids to break down.
A tech friend of mine finished replacing the third water pump on an owner's SL...the owner has only logged 8K miles on the car! The seals on the pump blew out from sitting for long periods and were allowed to deteriorate.
Can't comment on the 320, but in hindsight, my wife is the primary driver of the SL and as such, uses it as a grocery-getter, not a touring racer, so the V-8 is a waste for her purposes, in my opinion.
The M119 engine in the 8-cylinder series is pretty much bulletproof, and has a proven track record in other MB models.
What you need to consider is the overall expense of the R129 parts if something should break or fail. As is said earlier, the engine is pretty robust, but that doesn't include the ancillary units like the power steering pump, alternator, etc.
I haven't had problems in that area yet, but mine is still fairly low in mileage.
The softtop/hardtop hydroelectrical system could be a nightmare if a major failure crops up. Individual components run on the average of $300, but you could be in the low $1000s if an electrical CPU component should fail.
In fact, the SL is a surprising mix of cheap and expensive parts. Break the armrest cover slider latch, and it's $8. Break the dash cluster lens, and you're talking $327 (ask me how I know
).