Physically, there’s no problem with switching from run-flat to non run-flat tyres. You might even (almost certainly) find the car rides better than it did on run-flats and the Benz B-Class always had a fairly brittle ride, so any improvement is worthwhile.
Yes, you’ll have to arrange some sort of contingency should a flat tyre occur, but you can source a puncture repair kit pretty easily. There are even puncture-repair-in-a-can products (and have been for years) although they’re not always 100 per cent successful. If you’re keen, you could always find a space-saver spare from another model at a wrecking yard. Provided it’s the right diameter and the stud pattern matches, you’re good to go. Don’t forget to include the tools required to change a flat, though.
The other thing to do is to replace all four tyres at the same time. Driving around on a mix of run-flats and non run-flats is a bad idea and could give the car some evil handling traits, especially in the wet.