I can build you an efficient opencv c++ (and/or python) motion detection video recorder. The motion detection would be zone-based (like axis etc), with motion threshold, only records motion frames.
Relevant Skills and Experience
I have created opencv c++ (& python) solutions for motion detection video systems, with recording features as you describe specifically. Familiar with linux motion detection systems, esp. my own ones.
Proposed Milestones
$333 USD - first installation on your Pi3 of motion detection recording; no recording for non-motion, more
$222 USD - motion det. recording sys; testing, and any fixes or adjustments that need to be made
Additional Services Offered
$300 USD - multi target, person video tracking system, counting library, for Pi