In this case, I would actually agree that a tub might be better for you. Usually I like to have those people who like tanks stick with them, but a tub would have the ease of care that you seem to desire. I'm certainly not saying you can't handle a tank...but they can be tricky to maintain.

If you do want to go with the tub, there are some really good threads in the forums that give advice on how to set it all up. But if you really want to go the tank route, here's what you should do:

First, use the 50gal for another animal. Not even an adult BP would be happy in one of those, let alone a youngster. That 20 gallon you had her in is actually just right for one her age and size.
Make sure your UTH is sized right for a 20 and that it's on one side of the tank. Using two UTHs on a 20 is not overdoing it as long as they are both controlled. My tank is a 20 long and I have two UTHs and lamps for day and night.
Your most important EQ is going to be your thermostat(s) and digital thermometers. Also, you're going to want to cover the back and both sides of that tank with a nice solid background and some form of insulation to keep the heat in. I use foam insulation board from Lowes..really cheap for a huge piece cut down to needed sizes. With that, the screen top covered with foil as in the sticky, and your controlled UTHs you should be good to go. Lamps can then be used to boost a few degrees if needed. Required temps are 92-93 warm side no lower than 90, 82-83 cool side no lower than 80, and ambient no lower than 76.

Thermostats for heating devices are found almost exclusively online. The best place for a nice Johnson or Ranco around $75 is ReptileBasics.com. If you have a bit more money, the better buy is a Helix or Herpstat for around $110. The site has great pix and Rich is the best to work with.

Hope that wasn't too much...I just really like trying to help people get their tanks right if they really want to keep them.