The danger is that you risk getting carried away by the mass of equipment available for self-builders to install in their vans, and end up spending the same,or more,and expending a lot more time and energy ,than would have been the case when buying a ready made camper.There are online shops selling self build enthusiasts absolutely everything they could ever wish for.
So what level of tech to install?
An important element for me was to create a "stealth" van,i.e a van containing enough elements needed for reasonable comfort inside,while from the outside retaining the appearance of a basic white van so as not to attract attention when parked overnight in town streets and supermarket car parks.I wanted to avoid the many hatches and flaps,filler taps ,exit taps,vents and exhausts,double glazed obvious "camper" windows,and especially, swirly logos that you see on the sides of all commercially made and many self-build campers.
All camper vans need seats,and a bed;usually the one converts from the other.And cooking facilities:I wanted a kitchenette with gas hob,and kitchen sink,fridge and a microwave to add some versatility to the cooking (microwaveable ready meals,including frozen ones if the fridge has a freezer compartment, will provide a reasonable standard of cuisine!)
To ensure comfort while camping away from sites I want a separate washroom cubicle,with sink and shower,and featuring a cassette toilet for ease of emptying.For the kitchen sink i suppose you could just boil water in a kettle for washing ,up-but because of the shower/washroom I will need a proper water heater.
Finally I want some air heating as well ,to keep warm in cooler times of year.
Your under-slung fresh and waste water tanks,and gas tank ,will add to your fuel tank,cabling ,chassis members and spare wheel under your van to create a whole host of "no-go" areas for drilling holes through the floor above ! All this needs very careful planning.