The wonders of plumbing. May I ask though why you aren't just going to a hardware store or a contractor's outlet? Ordering pieces online has to be much more expensive than the local stuff. When I did my first plumbing job I didn't know very much, I just went in with the bulkhead and said I needed that to fit and how many feet I needed to cover. They'll be more than willing to help.
When it comes to the back of your tank, that is easy. I used a black oil based paint and a sponge brush on mine. Took two coats, but could have had three. The only issue with mine was that the tank and sump were up and running when I painted it, so I had to have stuff over my sump to prevent anything from spilling into the water column. I talked about it some when I did it in my build thread, which should still be in my signature, if you are concerned or confused. It might take a good amount of reading to find it. I don't even remember if it was a flat or gloss paint.