Experiences of *old* folding bikes?

Depends. Most UK Raleigh/Dawes shoppers have rims of different diameter (bigger etrto size 451, as opposed to 406 for most BMX rims - if you're unsure, check your tyres - it should say on them). This is not so much of a problem, as US Raleighs mostly came with 406 wheels. The issue is with brake drop, especially at the rear - it will lead to more flex under load and less effective braking. However, it is mostly not dangerous, as the new rims will almost certainly be better than old stock chromed ones.

Another couple of issues that are harder to escape are that most BMX hubs (certainly modern ones) are a different width from old shopper hubs + axle diameters are almost always bigger. You can cold set / persuade the frame and file out dropouts to accept them though. But they will need a fair bit of fettling.

You could be lucky, of course...

Tl:dr What have you got?