If UGVs have separate rotating turrets, how will the player turn the turret without turning the body?
Add something like Shift-RClick or Alt-RClick?
Dozen of combination you can get very easy. 3 UGV types (scout, medium and heavy) x 6 weapon types (machinegun, plasma, laser, cannon, grenade launcher, particle beam). Add here next-gen UGVs...
So, to make life more easy for user may be it's worth to add filters to market instead? Something like "show only compatible weapons/parts/equipment for..." It can be useful for aircraft too.
Making only predefined configurations make upgrades more expensive or non-obvious and force to buy unnecessary things.