I am expecting a MkIII.
I might model 169041, and paint it with Citadel Hex Wraith Flame.
A Mark 5 (change away from Roman numerals occurred during the war) is essentially a Mark 3 but with the co-ax swapped to a .30 cal Browning. The 5/1, the version that saw service in Vietnam is a Mark 5 but with the addition of a .50 cal ranging machine gun. So essentially hull and turret should be good to go if you want an earlier version.
The Australian Cents principally differed by having.
a) Bustle mounted stowage baskets.
b) Siting vanes on those tanks with the ‘B’ type barrel with the fume extractor.
c) External, demountable auxiliary long range fuel tanks (not fitted to all and not fitted all the time, there was a shortage of such).
d) The bazooka plates were removed very early on.
e) Attachment points for a couple of road wheels on the glacis plate (not fitted to all, or rather the road wheels weren’t always stored there.
Technically speaking, the Aussie 5/1 was better armoured, there was additional plate welded onto the glacis but you would struggle to notice in 1/50, it’s only apparent on close inspection on the real thing.
So if Rich does something similar to what he did with his 1/56 versions and produces an ‘A’ type barrel, bazooka plates and makes the basket, bazooka plates and auxiliary fuel tanks separate items ( I figure they will have to be anyway) then you should have a fair approximation of a Mark 3 or a Mark 5. You or he would probably need to make a glacis stowage bin and spare track. All of that I suppose is dependent on what the chaps commissioning it want to do. There are 1/48 Cents out there already if you want vanilla versions.