I am hoping to build out my system that sounds very similar to yours. I would not use PEX in the solar panel boxes, but I don't think that is your intention anyways (not how I read it.) I am also planning on running PEX tubing from some solar water heaters I bought on eBay (copper internals) to a single 40 gal water heater. I actually have a second one, but I am considering running a winter and a summer configuration. One of the water heaters is stripped and painted black. It will be my summer heater. My attic gets to 120°F in the summer here in Houston, so I may run the solar heater and maybe a car radiator or transmission cooler in my attic to try and cool the attic while heating my water. I have a second water heater that I have left insulated. It might be my winter tank. My attic was 40°F last night. I was thinking about a small controller that would take water from tank as long as it was warmer than the cold water inlet to the house.
I have an on demand water heater made by Takagi that I installed. I just love it. It can utilize pre-heated water and just uses less gas. I hope to pre-heat my water using solar from say... 80°F to 100°, then use the gas heater to bring it up to 120°F. I would theoretically cut my gas demand in half.
The big problem that I face with the thermosiphon arrangment is that the panels will lay on the roof, and the outlet should come off the top and go into the roof thru a pennetration, rather than go back downhill to get under an eave. THat kind of sucks. Ease of piping may preclude the thermosiphon route for me. I might have to add a small solar powered pump at a cost of about $100 which would take a long time to payoff in savings.