One important fact about the war wagon in WAB that I didn't see mentioned is that it can't charge. It's intended as a firing platform, and a defense against cavalry.
Now really, war wagons were already obsolete by the time period that the Empire is largely modeled on, since they could easily be destroyed by field artillery. But here are the rules that were used in the last warhammer-empire campaign list:
War Wagon: Costs 150 points.
A War Wagon is considered to be a chariot (and so is mounted on a 50 by 100mm base, as for all chariots), with the following exceptions: it can never charge or pursue under any circumstances, and may not chose to flee as a charge reaction. If forced to flee, it moves only d6". Also, it is not destroyed automatically by a wounding hit of Strength 7 or greater. It automatically fails Initiative tests. The War Wagon is Stubborn, and immune to Fear and Panic (treat Terror as Fear). It is a large target.
Any cavalry, chariot or mounted model of any kind that charges the War Wagon does not count as charging, and thus does not receive any charge-related bonuses (such as impact hits, the strength bonus from lances, or striking first). There is no combat resolution bonus for flank or rear attacks against the wagon itself (this applies to any units attacking the War Wagon, not just to mounted models and chariots).
The War Wagon has an armour save of 3+, Movement 5, Toughness 6, Wounds 6 and Leadership 8. It may make 6 Attacks in combat with Weapon Skill 3, Strength 4, Initiative 3.
It may fire 6 handguns at Ballistic Skill 4 (the handguns may fire even if the War Wagon moves, but the usual penalties apply). The handguns are considered to have a 360 degree line of sight for shooting purposes.