Why are the rear additional seats so expensive to buy
But whos the best people to fit and bolt them to the floor?
All VW original stuff is expensive. The vans don't have seatbelt fittings so you need the safely bolt through the floor a tested seat with integrated belts (see the rear of the seat/bed in mine. Or, use any old seats and attach belts to side pillars (hassle and not safety tested). Those rails you see on the floor of mine will take sliding quick release seats and you can also attach sliding QR seat belts to them (or just about anything else). Loads of places can bolt seats in though. My kids/wife will be sat on mine, so didn't want the Heath Robinson approach!
Boxelder what do you use to insulate and finish trim for the interior panels.
You can use all sorts – sheeps wool, builders types of insulation etc. For the trim, you glue (high temp. glue) on carpet/vehicle trim, onto ply-wood sheets. Mine was done by vanbodies as stated and they use eco insulation (recycled plastics I think) and vehicle trim carpet (thin, stretchy, hard wearing)
I used them as they offer things other firms don't. Great to deal with too (boss had a nice mtb in the back when I met him), they clearly more expensive than DIY.
If you (or anyone) is going DIY with a SWB T5, I have pre-cut plywood panels going for £nowt.