The Trafic / Vivaro etc crew van is actually a really good seat / bulkhead factory conversion. You can also get them aftermarket for a wide range of vehicles http://www.doublecabin.co.uk/index.htm
If you look, the seat is actually a good way further forward than the minibus, gaining a useful amount more luggage space (and bulkhead is much safer for passengers with a pile of bikes etc behind them). The seat cushion also lifts and there are a few access hatches so you can still occasionally carry long things. The bulkhead itself is made of plastic but there is a lot of steel reinforcement joining the belt anchorage points to the pillar behind the sliding door (so bulkhead as a whole is VERY structural).
I've run a van with a seat conversion and now a factory build combi. I've also done a lot of seat strength tests at work on converted vehicles / aftermarket seats. Lets just say there is good and bad stuff out there, and most integral seat / belt conversions need MUCH more than a few spreader plates under the floor.
I'm not even sure the real price premium is that big (factory vs conversion). Individual fixed M1 seats and belts (google Rescroft) are about £250 each (so £750 for a row of 3). You can make them removable by fitting to tracking but that is more ££ and the installation is CRITICAL (badly supported / secured tracking may rip in a crash). There aren't many legal fold up seats with 3 integral belts (google Scotseats - probably has the biggest range). The full Vivaro crew van conversion is over £2000 aftermarket.
Now I'm strapping our kids in the back I will happily pay the premium for factory fit seats, massively improved safety and no hassles with insurance etc. Some combis also come registered as cars so you get regular speed limits. Some van conversions can be re-registered as dual purpose vehicles which also gets you regular speed limits.