It depends. If I were in your situation, I'd have Unifi APs in both structures, so I could keep a consistent network between the buildings, though with that distance I guess it doesn't matter as much if you have a completely separate WiFi system in the barn.

The setup process can be a tiny bit tricky, but not too difficult. It basically tells the router not to handle the duties of your main router (like DHCP).

There are other systems out there these days, like Google WiFi, which could accomplish what you're looking for out of the box.