Re: Chicken or the egg?
I believe it's better to complete your van first (including all the interior and exterior components ie. cabinets, bumpers...). This way you'll know how much you vans weights (ideally at each wheels) and you will be in a better position to properly dial in your suspension needs.
Not all 4x4 conversion companies offer customized kits but some like Ujoint Offroad do and that's as good as you can get.
2001 E350 AmbuLand
2008 E350 AmboEvo
1965 Econoline Travelwagon
1970 Econoline popup camper
A few bicycles...and sandals ;)