I've read your post a couple of times. Don't know if I'm reading it wrong or something but I not sure if I'm getting it right, but I'll say this anyway.
Your looking to buy a tub and keep it in a tank and then move it from there to a snake rack ?
If you can't afford the rack yet then why not wait a little and get the setup you want before getting the snake?
If you don't want to wait then how about putting a tub in a wooden viv. Holds heat well and has vents so no need to mess about seeing how much of the screen you need to cover and what not. You can put the heat mat in the wooden viv and then the tub on top of it (or lifted off it a little if you want to). . Any thermostat will work as long as the probe is on top of the heat mat or between the heat mat and glass tank and not inside the tub. Although the probe can be inside the glass tank and underneath the tub since there's no snake in the glass tank to move it about in your case.
I would recommend using a digital thermometer as well as the infrared gun to have a constant temp reading of the hot side. Makes life easier.
As for temps a good thermostat will keep the temps in the tub pretty good as long as your room doesn't get to warm of corse.