Hi Carl. Rust shoudl not be an issue if it was prepped with rust inhibitor. We had a 1988 Mazda RX7 which we bought new and had Waxoyl put on at the dealer. The car never had rust even now as we sold the car. Simply amazing. By now, most Mazda would have been a rust bucket... but not this car. I can't see why it can't be done for UK vehicles either especially you know that it s problem in UK.
As for the MB, we have our fair share of rust too... On W126, they are notorious for front fender by the wheel wells and the back wheel well area where water get splashed into the crevice. Rust spot on rear window by the edge is also an issue... same goes for W124.
Late model E320 also requires premium fuel... so it is no help to you either. Diesel will get you 40% more fuel mileage than gas engine... so it should be considered.
Having another W210 would make it easy for you to take care of... you know what to do when problem arises. However, you are probably looking for a variety to drive... not a twin.
The biggest problem with buying older MB is the wear and tear items... shocks, tires, brake, steering, maybe rusted/broken springs...air conditioner... etc. Unless you are super handy on fixing car, these item will cost you dearly that you probably wished for newer car instead. Take your time to weigh all options... and yes, MB would be my choice of vehicle...