FWIW, the GoControl zwave sensor “essential” kits are marked down at many Home Depot’s right now so that you get three sensors for $25 (although one of them is a motion sensor). They’re still $45 on the website, you have to look for local in-store clearance.
They’re probably on markdown because gocontrol is intending to come out with a new Z wave plus model, or maybe just because they’re boxes that have the wink logo on them. But if you are interested in zwave classic sensors, there are good deals to be had right now.
But again, I think if you just have two zigbee networks you’ll probably be OK.