The threaded bungholes in the top of the drum are National Pipe Thread. The big one is 2" NPT. The small one is 3/4" NPT. If you need two smaller ones, get a 2" by 3/4" reducer bushing. Then the rest of the plumbing will all be 3/4" NPT, and probably a 3/4" NPT to Garden Hose Fitting, available at your local hardware store or plumbing supply house. Use steel pipe. PVC will crack, and the threaded black pipe will assemble easily enough and allow you to change things around if need be.
A valve in the vent will prevent any sloshing of the water from spilling out. You shouldn't need to caulk anything in place. A little Teflon thread tape will keep things from leaking and make them tighten down nicely.
If you go down to your local hardware store, I would think that with a little explanation of what you want to do, the salesperson in the plumbing department will be able to help you select all the parts you need.
I wish I saw this earlier while I was still at work. I'd assemble what you need and take a picture. I can do this on Monday if that helps, but I'd imagine that you'll be already finished with this by then. If not let me know and I'll post a pic next week.