Sherman Mk I
The turret has two problems. It is slightly too small, and is let down by a very spindly barrel. The Sherman's 75mm was a very solid gun, and doing no more than replace the barrel will give a very acceptable medium production M4, which is not a very common subject. If you have a Matchbox firefly turret in the spares box, it may be mated to the Airfix hull with a little care, by slicing the splash ring carefully with a knife, leaving it attached at the rear, and trimming the back edges of the raised areas by the driver's and co-driver's hatches. With care it is easier than it looks. Eliminating the gunner's hatch and cutting the barrel to the length of the Airfix barrel will give you a really good M4.
As usual we would advise building the hull BEFORE adding the bogies, as then it is much easier to get them all aligned. The track is a good representation of the early rubber block type. Sherman track should not sag, so when you fix it you will tend to strain the drive sprocket and rear idler. We would advise a beginner to strengthen both with slips of polystyrene or epoxy putty and make sure they are really hard before adding the track. (See similar comments on the Panzer IV kit)