I had new seats put in my troopy (was 11 seater - cant put baby seats in...).
2 single Recaro copies and a 3 seat bench (
www.techsafeseating.com.au/van-seating/safetyexcel) . All done by wesbarvanquip.com.au so legit roadworthy etc. (I still have all the original seats, so I can add the original 2 rearmost seats to add another 4 seats if required.)
They used the original seat slides but did a cut & shut to get some extra room when slid forward and mounted slightly further outwards.
Was $5k all up from memory (was nearly 10 years ago)
I also did similar to Fly in that I removed all the internal panels and copied them onto thin Masonite and added carpet. Cut in some doors to put a compressor in the panel space and create some storage.
Its all doable... just time and $.