The 12vdc thermostat would need to be powered by a battery or a power supply... Or a battery which is continuously being charged.
I would set the panels up in parallel. You had said 54 feet. Which at say 60VDC(12amps) using 10awg wire. the voltage loss is minimal. around 2.5% voltage drop.
so yea, the relay will be a cost. It will most likely need a heat sink. The solid state relay I just installed required one. I made it myself instead of buying one(scrounged).
It would be wise to put a circuit breaker of say 15amps in there. Along with an appropriate fuse for the 12vdc circuit. I didn't seem to find any affordable 100VDC relays at 15amps. 100vdc 20 amp SSR seemed to be more common.