If you can not afford the rack then you can not afford the snakes. You failed to mention anything about getting a tstat which is the most important thing you need to buy. Having a rack you are essentially putting all your eggs in one basket so to speak. Not having a proper tstat on a rack places every snake in danger. A good tstat is going to be $100+ so you need to figure that into the price of the rack system.

Your best bet for keeping stacked tubs at a good temp is placing them in a small room that you heat with a small heater.

Im with Eric and say get the housing taken care of before you start adding snakes.