Indeed many many topics on this subject, have a good search of the forums.
But remember whatever heating you go for, heating an un-insulated van is a bit like trying to carry water in a seive. The following might take you a weekend, depending on how handy you are, but well worth it:
Take your existing ply lining off and stuff the space with fibreglass loft insulation (polystyrene insulates better but is almost impossible to employ for this job). Obviously re-fit the ply afterwards.
Then you need to do a bit of handy work...get some 6mm ply and continue the ply lining up the rest of the sides of the van, and all the way across the roof, not forgetting to stuff it all with insulation.
On my van I replaced the existing liners on the back doors with one big piece of 9mm ply (stronger to screw stuff to) & 6mm on the sliding door. My van is a giant cool box now, when it is frosty and heated inside the only bits of frost that melt are where the cross-members go over the roof, bridging between outside & inside skins. About 200W of heat (with a mini desk fan to circulate) keeps the air temp in the van well over 10 degrees.
It is more than worth the hassle
p.s. on nights when it's only just nipping freezing, the residual heat from your machine will be more than enough to keep everything snug. With the effective insulation in place, it's only prolonged cold spells or over a weekend where you actually have to even bother plugging in the heater.
Also don't under-estimate the importance of circulating the air. You should be aware that different parts of the van will be at different temperatures. I lost a QD on the solution outlet of my machine to the frost, even tough I had a heater going in there. Obviously with a great big truckmount between the heater and the QD's, being so close to the outside of the van, the heat didn't get round there. I use a mini desk fan, very cheap off ebay (example
here)
Also bought a couple of cheap thermometers and put them in the coldest spots in the van - inside the side door near the chemical jug, and inside the back door under my solution hose reel. If you can keep those two thermometers at an acceptable temperature the rest of the load space is going to be fine.