This is too complicated a question for a short answer.  Some guys will
scuff up your old paint a little with a Scotch pad, do a half-assed masking
job, and hit it with a quick skimpy recoat for $500 or even less.  If
you're lucky it might even stick.  At the other end of the scale you'd
strip down to bare metal, do two coats of primer, wet sand smooth, a coat
of sealer, a few coats of finish paint, a couple of clear coats, wet sand
smooth again, and polish.  This can easily run $5000-7000 or more.  A van
is big and it takes lots of hours to sand and quite of bit of paint to
cover.

At a minimum, you'll want your painter to sand your van thoroughly so the
new paint will not be trying to stick to a shiny surface.   Talk to your
painter and find out exactly what he plans to do.  Even a schlock job can
look good for a year or so, but poor surface preparation will invariably
lead to checking, cracking, and peeling after a short while.  About $2000
should get you something that should hold up for a while, depending on how
long you plan to keep the van. At all costs, avoid the chain places that
advertise a low-priced "special". You'll get the schlock job described
above, done by bored kids making minimum wage.

Bill