A UTH will do almost nothing with the setup that you are using. The double glass and thick substrate layer will keep the heat from the UTH from getting to your BP. A UTH is completely unnecessary. As long as you provide the right temperatures, how the heat is preceded doesn't matter (as long as it is done so safely).

If you do get a UTH it must be regulated by a thermostat. The least you can expect to spend on a thermostat is $30, but a better thermostat will cost around $100.

I agree with the post above, that tank is going to be a nightmare to maintain. A tub, or PVC cage would be better. Even a smaller tank would help.