Hi mate
Ideally I'd replace as a pair from a handling point of view, (my opinion) but if u just want it to get through mot and are getting rid in the near future a second hand spring would do.
Shouldn't need to change top mounts, but you will need spring compressors to change over otherwise it's gonna be very difficult,
I'm a novice but just fitted a lowering kit, reckon I could do one in 30-45 mins which is probably a bit slow..
Just try and mark on the strut where the bolts are to try and avoid havin to get camber set again...
Good luck!