Some quick help please

If you are gonna replace all the gears on the m/s Then it would be a good idea to change ALL the gears including the input and cluster, on account of those gears are well broken-in together.
Whereas the three m/s gears when running on your cluster, may be noisy. The seller probably knows that, and if I was him I would let you make me a deal on a trade for whatever good parts you may have, in addition to the 3 gears and M/S.
I wouldn't be scarred of used synchronizer brass either, so long as they bite hard on the cones with light pressure, and sit reasonably high. Put the best one on second gear!