Why the van? Several reasons, really.
1) Easier for getting in/out with weapons than a sedan.
2) Blend in more places than a sedan would. A sedan in the middle of an industrial park may draw attention, whereas a delivery van in corporate colors might not.
3) Cargo. You can carry more drones on a van.
4) Body and Armor. If things go suddenly to hell, a Bulldog is a lot more likely to survive a fight than a sedan, especially if you up the armor.
5) Can fit the whole team, even if one of the team is a troll.
And as for why I don't just steal/buy a van for every occasion where I'd need one? Because that's more expensive, and it leaves a trail that is more closely connected to whatever crime you're committing. Plus, if you buy something just for the run, you have no time to go over it and check for things like listening/tracking devices, back doors in the pilot program, a killswitch on the engine, or even a car bomb. Don't trust a piece of equipment you haven't had a chance to check out.