Yeah, when power loss NAS can shut off gracefully when UPS commands it to. You set your NAS/PC to shutdown when the UPS enters "battery mode" from "power mode". IF you want to keep using PC and do the shutdown manually, you can disable this feature.
Get 2 UPS for what devices? Your primary one - the workstation power draw is probably going to be high so 1200 is safe. 650 to 850 might suffice, but higher is nicer to have - longer standby time in case of power failure. APC better built (atleast the outer plastic shell) but the battery is sexpensive when it go bad.