Side or top armor are good upgrades, basically magburner and extra ammo are decent ones, and possibly zoom for the gunner weapon
You don't really need a zoom for the regular gun, you can make some awesome long range shots without it
Also, upgrading the drivers gun is really not worth it, basically the HE and AP variants have such a small bonus compared to the regular one which can do well enough against both kinds of targets (although the AP one will 1 shot planes whereas the regular and HE will not so you may want to get it if planes are pissing you off)