The only part I can answer is to say you probably shouldn't use PVC with heated water. I'm not sure what temp causes PVC to have problems but when plumping a house, for example, PVC can only be used with cold water. For hot water you CAN use CPVC if it's available. It's rated for hot water and you can use it immediately out of any hot water heater. CPVC is significantly yellower than PVC.

The only problem I know of with CPVC is the transition to or from PVC--there are adapters.

AFAIK, while Sched 80 is definitely stronger than Sched 40, there's no useful advantage I know of to using it. I do have some Sched 80 ball valve quick connects but that was because I couldn't, at the time, find the Sched 40 ones. They work really, really well but were expensive.

For my solar panels, the only PVC in them is in the cold water feed to them. The heated water comes out of hoses and goes directly to returns.