Not bad & a good idea. Heres what I had in mind for the turrets for NTDs n mine HL2D mod we are working on for the turrets.
Lv 1 turret would be the original HL style turret with one barrel & small body
Lv 2 would be the dual 20mm cannon turret like yours but with a round enclosed turret that looks AI or remote controlled
Lv 3 would be the turret from HL Decay as that one is quite a bit larger than the older version & it has a mini gun like barrel IE 6 barrels & thus would fit the Lv 3 turret slot.