What you're proposing probably won't meet code, and carries a real risk of a steam explosion if allowed to stagnate. To just plumb it in-series with the hot water heater (no pumping loop) you have very little thermal mass in the panel system, so you WILL hit very high temps, and you get very little energy benefit without the water flowing when the sun is out. There are batch heaters outer there that can be plumbed that way, but they have a few 10s of gallons of water in the collector which both stores the collected energy and keeps the collector temps down, but most aren't suitable for places with high freeze risk.
If the aging collectors designed such a way that they will drain reliably via gravity (many are, some need to be mounted with a bit of tilt), it's safer to run them at atmospheric pressure, and build yourself an insulated tank where can use a big coil of PEX containing the potable water as the heat exchanger as pre-heat to your electric tank.
It would be similar to Gary Reysa's system, except that you have the benefit of starting with salvaged thermal panels. (He lives at altitude in Montana, so his summer/winter temperature issues are similar to yours.)