Definitely get a single passenger seat as you can make money on the downgrade. Watch for factory swivels though, as the seats lose height adjustment, which would undermine selling it on.Also, give some though to whether you want a bench or single passenger seat up front. Most start life with a bench seat, and fitting a single is easy enough to do, but finding one at a sensible price is another matter. So if you want a single, keep an eye out for van that already has one if you can. Swivels are an easy enough retro fit if you want them, but again if you can find a van that has them already, you’ll save yourself some time and money.
As suggested above, a Shuttle could be a good start point and will save you money on windows.
Swivel driver's seats are a bit marmite, so see if you can try one first, but speccing swivel means no bulkhead to remove in a PV.
On other points...
Sadly, factory sliding windows does not guarantee leak-free operation, so even that is not 100% the best route. In fact, adding windows later means a neater finish when carpet lining, as you can seal the carpet in when fitting windows.
Also, consider whether you want fixed seating or full rails. Kombi back seats are useful pre-conversion but more to strip out later if going fixed bed or full rails.
Kombi is still the sweet spot, I would have thought. At the lower end there was trendline with air con, but you'll just want to retrofit all the toys afterwards!