If you want rear leg room get a full size people carrier, take out the middle row of seats and use the 2 rear seats. It’s not flash but you aren’t going to beat it unless you go for one of those converted sprinters.
If you want an s-class, buy an s-class, just expect any money you save on buying a cheap, old one to be spent keeping it running. Go in with your eyes open, enough lube and the preternatural ability to hand it on to someone else if something really spendy contemplates breaking and you’ll only be ruinously fucked, not ploughed until your bones are dust.
I don’t know why you are fannying around, just buy one. Great car. Great value. You should be fine. Just find a good back street garage if anything goes wrong. You can always use second hand parts if needs be.