Probably better to not have bought a van. The people that convert them and fit the lowered floors etc have their own suppliers and finacial arangements with their own best factory option packages factory fitted (cruise, leccy doors and tailgates, certain engine types etc) that they have off the shelf ready made exhausts and body and interior panels for to miss the 10 inch lowered floors, same with fuel tanks suspension and many other bits. And they buy lots of vans at preferential rates.
There are also tax breaks for vans sold with disability related conversion works already done at least here. Its usually better and cheaper to get a new van from one of the big conversion companies (or a used already converted one) than to buy a used one first and then start converting that even if you can find someone that will want to do that. You MAY be better selling yours or not buying and taking a look at some of the advertisers on my site. EG an example:http://www.cummingsmobility.com/cum/Dev ... llVans.php
New and used converted vans.
Check VMI, Rollx, Braun, and others that actually do these conversions.
The big companies take brand new vehicles production line style and remove the floors, and strip the vehicle before rebuilding with new lowered floor, ramps, etc. This requires a lot of their own fabricated parts (exhausts, extended wiring, panels, interior parts etc) and as such they know the exact vehicle, and options that they need as a starting point.
Doing it afterwards is sometimes more expensive and usually more involved and less easy and difficult to do unless you have exactly the right vehicle and specification. Although it sounds like you may have!