Ideally you need to know how bad the floor welding is. It's ok them saying it's not bad, but that isn't always the case with sellers being honest - not that I'm saying they aren't, you just need to be careful.
The car itself loks ok, the inner arches will need some attention at some point. Pics can be deceiving so you need a real viewing of the car, but it looks honest enough.

Their £3k is around the right mark - but depends on close inspection and full spec. Swap? Depends on what value you put on the BMW as I have no idea.
They may not want a BMW neither.

My advise would be to go for a look if you're interested and go from there.