First items to be sensors. Overall best brand in low cost and good performance and easy integration.
Brand name or DIY AND COMPONENTS ( VIDEO ASSIST NEEDED.
FOR ELDERLY WIDOW INSTALLATION AND DISABLED LOCATION(S).
You’re not going to DIY a Z-Wave sensor 
Personally, I think the best are the Sensative Strips Guard. After that, take your pick of Aeotec, Fibaro, or other brands. My experience is they’re all much of a much.
If you don’t care about Z-Wave, as the body of your post suggests, then look instead at Zigbee and the Xiaomi Aqara sensors. Compact, cheap, and work.
In both cases you’ll need the relevant coordinator stick, HA integration, and non-battery devices to build your mesh.
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the neo coolcam door window sensors can be had in 4 packs for about €10-15 per piece on aliexpress.
Offtopic: I use them for other purposes by soldering wires on either end of the reed switch.
They use a CR123A battery (which if you have for example 100 triggers per day can lead to a lot of empty batteries), but I’m currently testing powering them with 2 rechargeable AA batteries in series instead. The voltage is on the low end compared to a single CR123A, so the sensor is reporting a low battery, but it is still working fine at the moment.