Well, most plastic tubs are not intended for reptiles or animals at allhaha. Little hatchlings do well in smaller enclosures because they feel safer. Wide open spaces will sometimes freak them out. I would suggest getting a 15qt tub. It will be a little big at first, but the snake will quickly grow into it.
The zooMed thermostat is extremely basic and still can cause temperature fluctuations. I have a really old reptitemp 500r that is still kicking, but it is a lot of work to keep it at the correct temp. I also have a hydrofarm which isn't too bad, but the temperature setting isn't all that accurate.
Let me look into that! There has got to be a thermometer out there that speaks the temperature.
I think its very doable to create the whole enclosure for under $300. What stores are you planning to shop at? Or are you going to do it all online?
Edit: look what I found!!!! http://www.amazon.com/Talking-Indoor...ng+thermometer