The van has a Thatcham II compliant immobiliser that is difficult to defeat without the key.
Depending on the type you get, it means stealing the keys means Johnny Scumbag still wont be able to start the vehicle.
There are arguments for and against, but if properly installed theres certainly no harm in it even if it only helps you sleep at night.
Do your research before buying though. I was a sergeant on a dibble autocrime team for many years and I'm still networked in (my replacement is one of my neighbours) and still get the latest news - it would seem that one supposedly well thought of expensive type regularly named on here has been sussed out by the villains, and recent reports in the media bear that out.
Also ensure the installer isn't lazy and disables multiple circuits, and not just the same ones for every vehicle. Old Johnny isn't daft and knows the common places to tap in to the vehicle and will look there first, so you want an installer with some nouse.
+1 for Disklok. Relatively inexpensive, and more than once I've seen cctv of villains approaching a vehicle, spotting a Disklok, shaking their heads and stealing a similar vehicle from up the road instead. Without doubt the most effective single measure you can take to immobilise the vehicle.
id personally also go for an alarm with proximity function so that it wakes up before theyve even laid a hand on the fehicle, and with a pager or SMS function - not much point in an alarm if youre outmkf earshot when it goes off.
Best of luck,