You can afford to buy an AP rack but cant get a quality t-stat? Im not trying to be critical of you but you could have planned that out a little better imo. A t-stat is the most imortant thing to keep your snakes alive and healthy. What are you using now for them? Why not just keep using the t-stat you have now while you saveup for a quality one. $30 for a hydrofarm is already 1/4 of the money you would need for a good one.
IMO on/off is perfectly fine for racks. They get the job done. Proportional is a luxury for racks, not a need. I would only say a quality proportional is needed for incubation since eggs are more sensitive than the snakes.