just curious about getting rid of the heat exchange....reason?
pex seems fine, what kind of fittings and are you planning to use the special pex for hydronic?
I thought when I was investigating my heater they discussed pex better suited for recirc versus straight plumbing.
Also switching to closed I guess you'll need a safety pressure relief...although if using the engine coolant the radiator cap really is it.
I would go with separate loops...maybe not all 4 on a manifold...but not all daisy chained either.
good pricing and great service
inexpensive booster pump
seemed like a good exchanger if you want to keep that sstem