First off, I buy all of my auto/truck parts at Rockauto and have for years; I've never had a problem. You're getting the same parts or better as any you get from a major auto parts store for around half the price delivered to your door. But, one has to know which parts to order.
What year is the van? How did the A\C fail, slowly not cooling over time (a leak), or suddenly? Is the clutch engaging at the compressor when the A\C kicks on? Too low a pressure (low on gas) will cause this; not enough to clear the pressure switch. Jump the relay to test. It would be best if you did all this before worrying about the compressor. Now, if your orifice or restriction device on the discharge after the condenser has trash, the most common cause is a compressor that has self-destructed for one reason or another.
Reading the thread, I see
@bullfrog is bouncing around the solution and the most common problems. In my opinion, and from Mr. Frog's informed posts, It's most likely a leak, and if it were me, I'd throw my manifold (Harbor Freight $60) and see if she's just low on gas, and if so, I'd add some to get her going again; then look for leaks. But you must have some gas left in the system to do this! A note here; you will have to add the die; the factory does not. Auto Zone has it and a ****** but cheap little rig to add an ounce to the system. They also have the pin light (batteries not included), so you can see it glow. Be very careful! Getting within three feet of this substance guarantees you can get that crap all over you and anything you touch, so beware! The most common place for a leak to occur is at the hose to pipe swaged connections. Run your fingers around each one to see if there's any oil residue.
A freon leak sniffer is $80 at Harbor Freight; they work great; I have one. You will need one to check for the evaporator leak. Put the detector in your vehicle with the A\C going and the windows up. You can use it under the hood after running the A\C but with the engine off and inside a shop. Work the tip of the tool around all the connections and condenser coils. If you have too much air moving around, it defeats the purpose. Now, if you have a leaking evaporator, it's in the HVAC box under the dash; need I say more?
If you want to learn to be a do-it-yourselfer and can afford the basic tools, you can do any troubleshooting and repairs you feel comfortable doing. If you don't feel comfortable doing the repairs yourself, you at least know what the heck they're talking about so you don't get taken to the cleaners. You can get a charging manifold and vacuum pump from Harber Freight for under $150. Whenever you open the A\C system, you need a new dryer; I keep spares; they're cheap. And by the way, most A\C systems today use 134 or something similar; if not, you can convert the system for 134 cheaply.