My advise on the sensors: Xiaomi Human Body sensor and Xiaomi Door/Window sensor. They work flawless with @TedTolboom Xiaomi app and last long on their batteries and they’re cheap!
(Prices vary a lot, You should be able to get a Human Body sensor around € 8,00 and a Door/Window sensor around € 6,00)
The app for turning it all into an alarm system: Heimdall 
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