Don't believe all the hype, Granville and get a cheap one sooner than later. There is so much tripe talked about them; I've even seen people in online forums calling for 4K resolution as HD isn't good enough.
Even a cheap (sub £50 - frankly even a £15) dashcam will record well enough to read number plates in 95% of lighting conditions. Given that we generally work in daylight that's just fine. Also, despite people's obsession with being able to see number plates in a foggy cave at midnight while someone's shining a military grade strobe light in your face, most of the time the dashcam footage will just back up your evidence in the event of a shunt so it just needs to be good enough.
Regarding battery life, etc, you have a leisure battery in the van. Wire into that and use it. Get a good, large memory card and Bob's your uncle.
I have one that is basically identical to this one:
http://tinyurl.com/ybhs2rhp - £40 and good footage, even at night.
Vin