A couple of K tanks on Ebay at the mo, looking at about £50 ea, but this could change. Difficult really with 15 tanks (depends on the condition) works out at roughly £80 per unit. If there are a couple of crinkle tanks there, then that would help a lot.
The CB750K set looks good, but again an aftermarket airbox and side panels can be had for not a lot of gold. The tank is another matter, pretty rare by now I guess. Paint looks OK though. (Have you got an early CB that may need this one?). I suppose it would depend on how much you need an early tank
(unless you are looking for a little investment)