http://www.megavanmats.com/ Good people, never realised they were in Leeds though… could have saved on postage!
I recently insulated. lined and carpeted my van, I don't know what you drive but here's what I did with my empty panel van (1982 VW transporter)
the load area had grooves (to stop a load moving around) I filled these with cheap carpet mat glued to the floor (works as sound deadening, insulates and levels the floor to prevent soft spots. I then laid 5mm ply down, and screwed to the floor to prevent buzzing. On top of this I put 3mm carpet underlay, then generic kitchen Lino.
Over the back of the van (where the engine is) I laid 5mm thick underlay, then rubber backed carpet. (this reduces the noise massively!)
For the walls/ doors I glued silver reflective insulation sheeting direct to the metal (you can buy expensive auto stuff but it looks exactly the same as the cheap stuff from hardware stores), covering as much of the metal work as possible, with the idea of reflecting heat generated back into the van rather than into the metal and off outside. I then laid a thin (maybe 1 inch thick) layer of household insulation foam (made from fire retardent fibre!) on top, making sure there was room for air to circulate (don't stuff it to the brim) Again, cheap hardware store stuff will be fine, but try to make sure it wont hold water, else you are creating rust traps against the metal and then hiding it!)
I then fixed 3mm fibre board over this (most people swear by ply, but my van had cut outs for wood lining and all the ply I found was too thick and wouldn't bend as well). I then carpeted over this with cheap black carpet (aldi boot liner no less!)
The van is now toasty warm! and much quieter than it was (important in an aircooled van!)
As for windows and vents, how confident are you at cutting big holes in your van? Perfect measurement, good seals and no changing your mind. Personally I'd pay someone else to do it, because I'm cack handed and pretty rubbish at stuff like this and i'd ruin my van, however, if you are confident and accurate it is quite easy and many places sell kits etc.
hope that helps!