Like I said, I use a 50W red bulb for my 15 gal. tank with a dome. The air in my home is usually 75/76F, so that bumps up Beau's air to ~80F, where it should be. You do NOT want that air to drop below 75F for BPs! That can cause respiratory illness, as well as other issues.

Definitely get your UTH and thermostat and set those up, and get a digital thermometer or two (if you don't already have two, one for both the wamr side and cool side) and see where it stands. You can also buy thermo spot guns, but that can be pricey. I got one from a reptile store for $35.
From there you can add in a bulb or CHE - whichever you prefer, that is up to you.
Like i said, trial and error. It's better to set up ahead of time like you have been, and then go out and buy more things after testing, than buying more than you need right off the start and never using it.