- What is an anti-aircraft vehicle?
That's a vehicle that can use its main gun to attack an airborne enemy. No special skill is required. It'd be great to have one, because airborne enemies are hard to hit with ordinary attacks. The Wirbelwind and Gepard both fit into this category. But, remember that the only main gun that can be equipped on these is an AA Cannon. - What do the numbers under main gun and sub gun mean?
All you need to know for now is that the greater the number, the stronger the weapon. - What should I modify?
The first thing to do is increase your engine's Max Load. Then, you'll be able to buy more Armor Tiles and install better (but heavier) vehicle parts. This will also allow you to modify with greater freedom. Next, try to increase your sub gun's Attack. After that, you can do whatever you want. - I don't know if it's better to buy vehicle parts or to modify.
One's not necessarily better than the other. There's a limit to how much a vehicle part can be modified. Sometimes, it's cheaper to buy a new part than to make extensive modifications. So, consider your options and save before you commit to one or the other.

















