trying to connect to Wi Fi hot spots while mobile is going to be next to impossible. The handoff just isn't fast enough.
If they want 'real time' video, you'll have to put a cell card in each car. The CATCH is, mobile streaming of IP video is going to absolutely destroy your data plan. (I'm talking 100 gigs a month). Unless you turn the video quality WAY down (like, to SIF) or if you can find an NVR that transcodes.
I used to be the mobile NVR product manager for a company. I spent a lot of time looking at mobile options. I only came across one product that would actually do it, reasonably, and that's the Servision MVG-400. It's analog though, and very picky about what cellular devices it uses. IT gives you audio, and GPS tracking though, which is cool.
If you want to retrieve the video afterwards, or just have automatic offload via Wifi when the cars 'get home' then your options get better.
What should the cameras cover? Well that depends on what you're trying to protect. I worked with a towing company. They wanted 2 things.. A: to watch driver behavior (so we put a camera facing forward out the windshield) and B: to prove to their 'customers' that their vehicle wasn't damaged by the tow truck, so we put a camera facing backwards.
If you're thinking of putting cameras in the CAB of a limo, I think you're going to run into privacy/legal issues..
I'd love to hear what other people have done for mobile deployments..