You are correct to recognize the underfloor heating system as a potential problem. My understanding is that it is not the PEX water pipes that are a problem, but the foil reflective barrier under the pipes. There is a huge possibility that you will need to pass an ethernet cable between floors to connect WiFi access points in the two floors.
This is a horrible time to be purchasing a WiFi system for a 4,000 sq. ft. house. Every manufacturer is releasing systems that support the emerging WiFi 6 standards, but there are no comparison reviews available yet to point consumers toward what to buy.
Here are recent articles about new WiFi. Notice the lack of detail and comparisons:
https://www.cnet.com/pictures/meet-the-wi-fi-6-routers-that-support-802-11ax-wifi/33/
https://www.androidcentral.com/best-wifi-6-routers
Back when I decided my Netgear Nighthawk was not "covering" my home the way I wanted, the Netgear Orbi RBR50 was the "only game in town." Today....??????
On this forum, we tend to blast Netgear for perceived faults and shortcomings. We own the Netgear Orbi. That's what we use and know. We know all the problems. Faults and shortcomings in other brands? No idea.
Unless the WiFi access points can be placed in almost the exact center of each floor, my guess is that you are looking at a "mesh" system with 2 or 3 satellites.
