Some more questions...

This AC gauge set that I rent from AutoZone. I see that it has two sides... for high and low, I assume. Is it just a matter of connecting it up and what, exactly, am I looking for? That is, what should a properly functioning system read out as for each side?

Yes, High and Low. The hoses are quick connects, much like air hose connections. The basic rule of thumb for the gauge readings is:
-High Side- approx 2.5 x the ambient temp in front of the condenser.
-Low Side- Ideally, about 26-28psi.
There are lots of variables that effect the actual readings. Ambient temp, humidity, sun load, cabin temperature, engine speed, just to name a few.

The accumulator... Do I need to go Motorcraft or is there another brand you can recommend that will do the same for less $$.

Like Mark said, aftermarket accumulators are just fine.

This is a purchased used 1999 van so not wanting to plow tons of money into it for the few years I may own it.

You won't know how much the repair is going to cost until you find the leak. BTW, does the van have Rear AC?

Still need to buy a UV lamp and check for the leak...

You might be able to rent that, too. Don't forget the yellow glasses. They really make the dye stand out.

Just a note:
-Along with the gauge set, you will need a vacuum pump to remove the air from the system after fixing the leak.
-You will also need a can tap. You can't use the cans of refrigerant with the charging hose attached(They all contain sealers and other snake oil that you don't want in your system anyway). The correct cans of (virgin) refrigerant are usually on the bottom shelf and have a screw connection on the top. The tap screws onto the can and the center hose of the gauge set attaches to the tap.