A standard Herald box won't mate with a 1500 box, the later has 20 splines on the input shaft and the clutch plate is bigger, therefore change of flywheel. I fitted one to a previous 1300 Dolomite but with a 1300 clutch plate. As far as I have researched the first Dolomites were fitted with 3 rail boxes before going to single rail, was that the same for the 1500 Spitfires?
In terms of MOT's my Herald Estate was taken off the road in 2010 ish and the DVLA have it as no MOT history even though I MOT'd it just before I layed it up but they still class it as tax exempt etc. I have kept it SORN'd It also has twin 1 1/4" Strombergs which I fitted, so anything goes with carbs.
As is said above, get the present owner to MOT it, if it fails on something major walk away.