Fabulous cars, phenomenal engineering from four decades ago. Powerful cars, a 500SEC having 260 bhp to move that big body around.
Rare, interesting and FAR more expensive to buy and run than the C215 which is "just" 15-20 years old.
Echoing abs (above), unless you can clarify budget you're going to get a disappointing mixture of answers here.
A superb, 35 year old C126 on 100k could be yours for £40,000, if that's your budget, but that's probably not what you have in mind.
£15k will get a tired 125k car in ordinary cosmetic condition with gremlins lurking, and rust emerging. Is that what you're hoping for?
Maybe you want to pay £5,000 for a car, and then spend £20,000 on fettling it over 2-3 years, using your own engineering skills ? Satisfying but takes time and patience.
Let us know what you're hoping for, and we may know someone with something that suits.
In the meantime, here's an old buyers guide by Classic and Sports that's well worth a look.
Click through to this link:
Mercedes-Benz SEC (C126) buyer’s guide: what to pay and what to look for | Classic & Sports Car