When considering buying a cheap motocross van or dirt bike van I'd recommend one thing - go for as reliable a van as possible within your budget. After all, getting to the race meeting on time (or at all) is as important as remembering your helmet, right?
I've had a lot of different cheap moto x vans and conversions, a Mercedes Sprinter, a high-roof Ford Transit, Renault Master Ambulance and a heavily converted Iveco. My Kentucky Estro lasted about a month before it was written off on the way to a meet. I'd say the better motocross or race vans tend to be spacious, something you need when doing, say, a wheel change on a tight schedule. They can be bought ready kitted out inside or a blank canvas for you to start your own layout, the choice is yours. A toolboard is nice to have and somewhere to sit down, shelter from the rain and take off those muddy boots (or tend to the injuries) is a must.
Cheap motocross vans
If you're starting out, a cheap motocross van will give you a feel for what you need and there's little risk as they tend to hold their prices if bought at auction, some even making more money when sold on again.