I have a 1989 300SEL with 131,000 miles as a daily driver and I’ve been very happy with it. I seem to get about 21-22 MPG and I don’t think the V8’s can get close to that. True it’s a bit of a dog, but I rarely notice it as I pass the gas stations and enjoy the spacious interior. Same engine and engine management system as the 300E so there are plenty of used parts available. Also, I feel the six costs less to maintain then the eight’s. No hydraulic rear suspension to worry about as on the V8 126 cars. As for value, around my area Minneapolis/St. Paul they would ask $3500 to $4300 for a good 300SEL with similar mileage. There is what looks to be a very well kept one currently on Craigslist for $4200 with 134,000 miles. There was recently a 1991 300SEL with 88,000 miles advertised around here at $6700. As a previous poster alluded to, and I’m sure you are aware, maintenance and upkeep are everything when buying a used Mercedes. I would think at 134,000 miles the head would need, or soon need freshening if it hasent been done. If the A/C is out, or weak, you may be looking at an evaporator core and a big costly job. Also, I think the ’88 was the last year for the older style interior. I know it’s a personal preference, but I don’t like the old style seats as much and it may bring the price down.
Good Luck.