The following message bounced from the e-mail address given in the previous post, so here it is for general consumption.
I bought a 1991 300 SE in April. I looked at eight W126 saloons/sedans and drove most of them. Given that the difference in the driving experience between the 300 and its V8 siblings is only slight, I checked out parts prices and consulted my regular mechanic, the conclusion being emphatically in favour of the 300 -- parts down to half the price of equivalents for the V8s, simpler engine, easier/cheaper to service. It has plenty of power unless you like to burn rubber, and the engine note is as sweet as they get. If you prefer the V8, from everything I've read (quite a lot) the 420 is the one to go for.
I have spent a fair bit on it as an investment in long-term ownership: body and engine dampers, engine mounts, gearbox mount, brake disks and pads, propshaft couplings (cracked at 130k), a front near-side ball joint (a common weakness, apparently, no doubt exposed by banging the wheel over sidewalk kerbs), plus service parts and lubricants. There is a slight leak from the radiator and a slight loss of oil; I'm hoping to persuade the after-market warranty company to pay for both, the latter probably requiring a head-gasket replacement, although I guess that it could go on for many thousands of miles without either being fixed. Whatever you do, stay away from dealers; there are enough trustworthy independent mechanics around. It's dealer labour rates that make owning MBs expensive; they are running at UKP 100+ per hour hereabouts.
The well known weak spot of the model is corrosion at the base of the rear window. Get inside the boot/trunk and look up at the row of circles. It can be fixed, but there are plenty of others around (they built over 800,000 in total).
It is a great pleasure and an interesting contrast with my W124 Coupé. I love the perfect proportions of the body (SE, not SEL). I get 25 mpg (UK gallons; about 21 per US gallon) from it. I am pleased that it does not have four items prone to expensive failure or replacement costs: airbags, catalytic converter, aircon (plenty of reports of failure on this site) and electric seats. Most people want all the toys, but I'm not interested in resale value, I am interested in maintenance costs, and I know from my W124 how irrelevant electric seats can be (especially without a position memory). If I get a lottery win I would consider installing aircon, and I'd probably give it a total respray, although the paintwork is generally very good apart from the inevitable chips and patches. I tend to keep my vehicles a long time, and at the moment I cannot see myself selling either of them.
Here are some good sites:
Hope this helps.