Power-vented hot water heaters are designed with stack temps low enough to run through PVC (or CPVC or ABS) venting rather than B-vent or single-wall vent piping to eliminate flue condensation concerns. Condensation in metal venting eventually corrodes through, but PVC et al are tolerant of the mildly acidic natural gas exhaust. I don't know of any power-vent units that specifies anything other than plastic venting materials. If the metal stack were all inside of conditioned space the risk of condensation would be low, but if it's passing through a cold garage or attic the condensation risk is pretty real in a MI climate. So, answer #1= "nope".
The general combustion supply air requirements are the same whether it's an atmospheric drafted water heater or a power-vent when the power vent is taking it's combustion air from the space rather than ducted in (direct-vent). But if the current supply air is too small for the combined furnace + water heater combination (which it might be) it's a good idea to bring it up to spec. The installation manuals for each would tell you how much is needed, but add the grill areas together. If you install a sealed-combustion direct-vented water heater it's completely isolated from and doesn't interact with the combustion air draw from the furnace. There are a few direct-vented HW heaters out there (some are even sold through box-stores), and that would resolve the combustion supply air issut at least as far at the hot water heater is concerned, but it's still worth downloading and reading the installation manual for the furnace to make sure it's up to snuff.
In some locations (not sure about MI) the subsidy for high-efficiency water heaters make them quite competitive with non-condensing power-drafted water heaters. The smallest condensing A.0.Smith Vertex has a burner ~2x the size of most atmospheric drafted versions, which may or may not require upgrading the gas plumbing, but it's standby losses are a fraction of the losses of an atmospheric drafted tank, and it's raw combustion efficiency is substantially higher too, and with subsidy it's likely to pay off eventually.
Since it's just you and the missus, a heat pump electric water heater might cut it too. Few states/utilities will subsidize it unless it replaces an electric hot water heater, but in my area the subsidy is close to 100% of the list price of the heater, making it CHEAPER to install than a standard electric HW heater.