Are you currently using the EFM's coil for hot water for the house?? This will make a difference on the plumbing.
In my experience, you don't need to spend the extra dollars on a brass circulator. They usually cost 3x or 4x as much as an iron circulator. I've had iron pumps running in open [lots or oxygen in the water] systems for over a decade and never had an issue. My water is not acidic or base, and is softened.
The cost of circulator pumps on ebay is about $50 for a small one, like a Taco 005, 007, B&G NRF22, or Grundfoss 15-x models. I would use what you can, and if it goes bad in four or five years you are still money ahead. They will go bad only if not running. LIke if you left it off for a summer.
You can also put isolation valves on either side and remove the pump let it dry, put it on a shelf to prevent corrosion if you have bad water and will be letting it sit, not running. all that happens is the surface of the iron housing gets a layer of rust on it, and this takes up the clearance to the pump impeller. Just a rotary wire brush will fix this.
Let us know if you are using the EFM's coil already so we can describe a plumbing circuit. Look for Traderfp's thread on plumbing a water tank, it has lots of info and drawings.