Questions about the ARS rack systems...

I'm looking to purchase my first rack, and I want to go with these, as they seem they will last myself and my collection a lifetime. I also just didn't care for how DIY racks looked.
I just have a few questions and I can't find answers on their own website.:(
I guess, I just lack understanding of how these racks work. How do the thermostats work in these things? I see they offer multiple thermostat options, I know of the herpostat brand but not familiar with others. If im going for a complete system, which is best? I assume multiple levels have to be regulated all on the same probe?

I've also seen videos of people mix-matching their systems with different sized tubs. I originally wanted to shoot for the 80 series, I keep mostly ball pythons and this seemed the best option to allow me to still decorate their enclosures and keep them comfortable. But after seeing a video displaying the different tub sizes, I realized mixing in some 70 series would be a good idea. Is it feasible to buy 4 levels of the 80 series, and then also buy 2 levels of the 70 series and stack them? (both have the 55in frame). Or do I need to buy a full 7 level system of one size?

And lastly, but not as important to me, do they do any sort of payment plan? I'm prepared to pay in full if needed, but a payment plan would make my life so much easier.