Before you start looking for a van, you need to know what you intend to do with it.
The van you buy for weekend trips with your partner will be completely different from the one a family of four needs for a six-month tour of a continent.

Let's look at a few things you might need to consider

Day van
Awning, side windows, bike rack, roof vent = Obviously a camper - possibly more of a burglary target.
For a single person or couple → Short or medium wheel base.
For a family of four → Long or extra-long wheel base. Perhaps a jumbo van, if the roads where you will travel are big enough.

Longer wheels bases, for example, won't affect must on the open road and can be great for wilderness getaways, but they make parking practically impossible in busy cities and they cost more to run and insure, plus their size may limit you to specialist garages for servicing and technical exams.

If you're not very tall, you don't need a tall roof, and that's going to open up the possibility of multi-storey and underground parking, which are not available to high-roof vans. If you need a low roof for parking reasons, be careful about which roof vent or air-con system you install, as some can be quite tall. Write the van's height somewhere visible for when you encounter height restrictions. It's easy to forget after a while.

Will you be working long term from your van?
If so, you will probably have valuable items with you and may leave them in the van when you go out. If you will stay at paid camping resorts with security, that might be no issue at all, but if you'll be parked in a busy city and leave the van often or for long periods, you definitely need to think about a van with no side windows, and consider adding extra security like deadbolts, side door reinforcements (in particular), poly-carbonate glass protection, etc.. All these things add time and expense to your build and determine in some way which kind of van will work best for you. Be aware that thieves may watch vans for a while to learn what's inside, when you leave and for how long, and so on.

In addition to your intended use of the van, you must consider your budget. If you aren't worried about the cost, by all means, go and buy a ready-built camper or have one custom made. If you are living from one paycheck to the next and your van is your home away from the burden of rent, you will do a lot of the work yourself and probably won't be buying anything spectacular to build on. In the latter case, you'll be looking at the most reliable model you can find, with the smallest engine, cheapest or most available parts, etc., and you'll need to spend a good while looking to find one that's good enough for what you want in your budget.

In Europe and Asia, German and Japanese vehicles are the most reliable and parts for them are usually easy to find. In the Americas, Dodge and Chevrolet seem to be good bets. Elsewhere, I can't say, but you only need to look around you. The makes you see everywhere are the ones whose parts will be easy to find and garages will know how to work on them.

See the next page for some suggestion of things to look for during your van viewing.