For me the engine and gearbox was not a concern (and ended up with 102bhp 5 gear) but single slider, tailgate, highline, twin captain seats all was.
Single slider because I thought this was important for a side kitchen layout, turns out it’s not so critical and have seen loads of side kitchen vans now with doors that fully open behind the kitchen which looks quite nice.
Tailgate because this seemed the best option for a camper, not sure this really is critical (and is a nightmare to search for on all websites!) but very glad I have one, I think it makes it a lot more of a proper camper and good to hide under in the rain, many would disagree as there are advantages to barn doors though.
Highline because this gives you a free pass for some of the other things that make living with the van easy (ac for example), poverty spec vans can easily fill the requirements you need but you need to be careful when reviewing to ensure the creature comforts you require are available.
Twin captain seats (with oem swivel) because this gives easy access to the back and keeps the missus happy with a comfortable seat (she always complained of being uncomfortable on long journeys in my cars). This is a difficult and expensive conversion as the single passenger seats are a bit like hens teeth so if this is something you want worth looking at in the base vehicle.
Lots of people say LED headlights is an essential too, these are great and the standard are very poor but I didn’t have them on my base van (since upgraded to THQ v3 which are marginally better) but I didn’t see them as a show stopper, and still don’t, after all the van (although is my only vehicle and so technically a daily driver) is not driven daily but many reserved for leisure
