So, not got the NAS yet, but deffo next.
Went and bought a refurb retina with all the goodies, and a big flash drive.
iTunes remarkably good at managing libraries on remote disks, so actually this is much easier than thought.
Also, rather than having years of lightroom libraries on local HD, just keeping current year -1 on there and mov ethe rest continuously to remote disk.
Seems to be working well, and by god that macbook is an absolute ripper.