I use 1/2" foam board insulation to wrap the back and sides of my glass enclosures. I also use it on the top screen with a hole cut out for the heat lamp. You can buy a 4'x8' sheet for about $15 If I remember correctly. It helps immensely with heat and humidity control. It also helps out with the electric bill since the heating equipment is running more efficiently.
As for a tstat, you're going to need one, no way around it. Since temps are dropping quickly, you'll need one right away, so get what you can. You can always upgrade later.
Hopefully somebody with experience using a rheostat will chime in, I just have never used one.
And just a thought, but it's important to have some money stashed away for emergencies. Equipment fails, emergency vet visits are always a possibility, etc... so once you have your new equipment try to stash a few bucks away when you can (I know, not easy sometimes. Trust me, I get it, I don't make much money in my line of work). But it's important and it would suck if something happened and the snake suffered for financial reasons.
If you need help with the foam board insulation thing let me know. I'm happy to help.