Although I'm curious why you didn't make sure you had everything set up and optimal for your bp before you got her, I think it's good you're doing your best to try and correct things asap.
Are you worried about ambient temps as in it's not high enough? A UTH will not provide ambient temps, they only effect your surface temps/hot spot. I think before you need to worry about upgrading you UTH size you need to get it regulated on a thermostat asap, so let that be your first priority next purchase. Everyone else already linked you to the best options for your budget so wishing you luck on everything!