The first part about cameras sensing movement and recording onto a NAS is basically the basic functionality of every NVR and IP camera setup.
You can buy 4MP IP PoE cameras for like 50€/piece, a NVR with a PoE switch for ~100€ and then you'd need a NAS. The NVR's firmware's always include motion sensing software - the ones i have have email notifications/SNMP/etc. built-in so all you need to do is set up a email account.
If you'd like Alexa/similar integration, that's where it gets tricky. You'd have to program it all by yourself since most NVR's are not Virtual assistant compatible.
My best bet would be setting up the above mentioned system, then you could have a bunch of tablets around the house that would start displaying the video that is being recorded to the NAS once the cameras sense movement and then you could integrate that to a web interface and add buttons that would, for example, turn on lights or sound an alarm or something.
Also, everything i mentioned so far would be relatively easy to make. A lot of NVR's actually have alarm sending capabilities so you can literally get some kind of a notification once the NVR starts doing stuff