I'm no plumbing expert but I am facing a similar situation right now. We are remodeling a 4300 sq ft single story rancher. Our temps get up to 110F in the summer and down to 25F in the winter.
All water heaters (WH's) do the same thing - they take the incoming ground temperature water and make it hot, usually up to 110F or so. You live in Wisconsin. I imagine it must get to below freezing there in the winter. Let's say in the winter the water gets down to 40F. If you want to heat that water up to 110F, that's a 70F rise in temp.
If you look at gas tankless WH's, they are rated at gallons per minute (GPM) at a certain temp rise. Take the Bosch C1050 ES NG for example. It's rated for 10.1 GPM. Sounds great right? But wait....that 10.1 GPM is for a 35 degree rise. So if you want 110F hot water, this unit will give you 10.1 GPM if the ground temp water is 75F. Is your ground water 75F during the coldest part of winter?
My point is these tankless units will put out less and less flow as the groundwater temp drops and the unit is asked to heat the water from a lower temp. This Bosch unit is rated to deliver 3.9 GPM at a 90 degree rise. All of a sudden it doesn't look that great does it? And this unit costs almost $2k!
My master bath shower has 3 shower heads that each flow 2.5 GPM for a total of 7.5 GPM if they are all on. That means this Bosch unit will not deliver enough water on a very cold winter day here. Two units hooked together would barely cover it. And what happens if one of my kids runs their shower or my someone turns on the dishwasher. Uh oh.
So what I've decided to do is go cheap. I'm getting two Bradford White tank WH's. These bad boys are 75 gal each. So I'm going to have 150 gallons of HOT water right off the bat in reserve. And each unit can produce 82 GPH of hot water at a 90 degree rise. That's 164 GPH for the two of them, AFTER I've exhausted the 150 gallons in the tank. Somehow I doubt I'll ever run out of hot water.
BTW, the total cost for the two WH's is less than $2k. The tankless system would have cost me almost $5k.
Hope this helps.