If it's done well vans still look good, just dont do the pimp my ride thing & put a flippin wing on it, ok? :P

Main thing, WHAT ENGINE??? Kinda need that info to get a better idea of what size pipes were lookin at here.

I'll give you a generic breakdown that should work well for a street driven 4-6 cylender engine. If it's got a V8, just bump it up to 3" all the way back.
I HIGHLY reccomend you get this done at a reputable muffler shop, have them do a full system for you & get a warranty on the parts. I do all my own work, but I still shell out the $$ to have the pros do things like this.

You'll need:
2.25" high flow catalytic converter
2.25" flex joint
2" x 24" Cherry Bomb glasspack
2" direct flow muffler

Have it plumbed all the way back from the down pipes to the tip with 2" pipe. The diameter is good to keep the gas velocities up which makes for good low end power, but big enough to breathe well for highway driving.

Mufflers:
Now if i remeber right, the van uses a myffler under the cabin & the pipe goes up over the rear axle & exits the side. Not much room for a muffler out back. I'd say off the top of my head you'll need something like this Magnaflow 5x8" offset oval

If the Mags are too $$, there's plenty of good muffs out there, just stay away from the wide open ricer knock offs & you'll be OK. A 2.25 turbo should be fine & will sound nice & mellow, yet have some thump to it.