If price isn't a concern and they don't need anything specific like POS integration, for example, I would recommend unifi protect with AI cameras (AI in the model number) and an AI key. I would also recommend dedicated hardware like the UNVR or UNVR pro.
Unifi protect now offers continuous archiving so you can set detections, rules, etc and the video will automatically be offloaded to the 'archived' device. I'm currently testing this with specific detections (face) and sending that video to the NAS. So far it is working well.
If they have specific requirements, for example, monitor rotation/tours, multiple video clients running on a dedicated PC, for a monitoring station/security guard/etc then I would not recommend protect.