Hi,
You must be leading a good clean life to have such good fortune
Not all commercial panels can be used in a drain back system. Some use a serpentine tube arrangement inside the collector, and won't drain back reliably -- these are not as common as the vertical risers with manifolds, but you should check to make sure.
You may be able to see the tubing through the glazing, or if the manufacturer has a website, check with them.
Assuming the collector uses the vertical riser tubes with horizontal manifolds, you can probably use it for a drain back system. Chuck Marken's Home Power article on drain backs recommends using riser tubes no less than half an inch in diameter -- so there would be some risk if the collector has small diameter riser tubes.
I like drain backs -- to me, if you get the plumbing right, then they are close to zero maintenance. You don't have to worry about checking or replacing the antifreeze.
They are also simpler and less expensive to build with fewer parts.
That said, the anti freeze systems work fine as long as you look after them.
If you use the drain back, just be really careful that all the plumbing and the collector itself have slope back towards the tank -- about 1/4 inch per foot is good.
If you go with the antifreeze and heat exchanger, you would effectively have a double wall heat exchanger with the two coils, but I would still use the non-poisonous propylene glycol instead of the car antifreeze.
You mention a 500 gallon tank -- that's a whole lot of tank for 72 sf of collector. Beyond a point you really don't gain anything by making it larger -- it just costs more to build and loses heat faster. I think that a couple hundred gallons would be plenty.
I'd not use the heat exchangers unless there is a clear need for them. My space heating system has no heat exchangers in the tank. The collector pump pulls fluid directly from the tank and returns it directly to the tank. Same for the floor heating system. The systems that preheat potable water will need a heat exchanger, and (so far) I'm quite happy with the large coil of PEX immersed in the tank. Its simple, and it gives you 100% efficient heat exchange for the 10 gallons of water that is already sitting in the pipe coil.
Gary