A galv chassis is not illegal to fit and doesn't need to be a q plate.
I just had one fitted. You then write to your local DVLA garage they book you in, check the chassis and stamp it with an id number. If the chassis is a like for like swap (no mods etc) then they allow you to retain the points for the points score of the vehicle.
I think it is 10 points you need to retain your old plate. The chassis gives you 5, axles 2 etc etc.
The problem with buying a 2nd hand chassis is as said, if it has been declared a right off and needs to be destroyed, it is not only because of the fact of the identity of a previous vehicle but also potential damage to the chassis. A breaker can sell them on but I would be very careful as technically it is as good as scrap.