A turret is possible without source edits. It's possible to get where the player is looking (bots, /selectobject) , freeze an animation in a certain point (Rotondo's Round Cards) and fire a projectile in a certain direction. (Traps, Turret Brick)
Combine these and a working model and you have a tank with aimable turret and no source edits.
EDIT: Can't be bothered to code at 8:49pm, have some psuedocode and MS Paints instead
EDIT2: Tank treads aren't impossible either, use invisible wheels (like skis) and an animation played when moving/fast/stopped like Stuntplane.