HiHi Dan, I have used Dynamat for roof and floor, and Dynamat superlite for side panels (u can get some good deals if you shop around). Not the cheapest option but really pleased with result.
On top of that I put 10mm polystyrene sheet, and then finished off with the double sided foil bubble insulation u get on a roll (2 layers). This was in a splitscreen panelvan. So U may not need to double up in a T5.
and finally the board panels.
The kit Westcountry Campervans sell looks like good value. What is the soundproofing insulation roll ? any pics? what is the material?
Good luck with soundproofing. let us know what u decide on, as I will be doing our T5 soon.