When you make the M60 Patton, you should make it with an interior. There are only two tanks with interiors available (both WW2 German). Not only will people then buy your M60 when they want an M60, they'll also buy it when they need a modern tank interior (to stand-in for whatever modern tank they managed to get an exterior for).

Also, with the M60, I think you should make it like the physical model kits that include extra parts for different versions of the same tank or aircraft. In this case, the two versions I think you should include would be the pre-1977 version of the M60A1 and the Gulf War version of the M60A3. (Of course, the interior changed between these models, so it may not be feasible.)
The M60A2 was entirely different, mounting a gun-missile launcher rather than the 105mm tank gun, and would require too many changes to include in the same model.

I mention "version of" these models of the tank because each one (eg the M60A1) was updated and changed considerably while keeping this same basic "M60 A1" designation (hence longer titles such as "M60A1 RISE", "M60A1 RISE PASSIVE" and so forth, as further improvements were incorporated).