I was thinking about using PEX, connected to the DHW on my oil burner,
but my roll of PEX is not rated for the temperature.
As for the pump, maybe you can just look at the pumps you already have and get something with similar specs.
When one of my pumps went bad, I looked at the type of cartridge pumps
on a new system at a neighbors house and got the same one for my system.
Years later, I replaced the other old style pump with another Taco cartridge pump.
Kinda looks like this one.
NEW TACO 0011-F4 CAST IRON CARTRIDGE CIRCULATOR PUMP | eBay
My system already has two pumps, so if I wanted to add another,
I would check out the wiring on the existing pump relays to see
how I could add on one more pump..
If you wanted to avoid buying another reply box, maybe you could
hack a connection, using a thermostat to drive an SSR, controlling the pump??
That way, you could use a neat little DC power supply, instead of having
a hot-running & humming 24vac transformer burning up juice 24-7.
(I leave mine powered down, until backup-heat might be needed).
The only problem might occur during a burn-fail and allow your add-on,
to pump the boiler temperature down to a very low temperature.