The plot thickens... The MBP is only accessing the NAS for TimeMachine backups. If the iMac is running when the MBP would be doing a TimeMachine backup, then you can do the backup to a drive attached to the iMac. In this case there is no reason at all to buy a NAS. Just use external drives on the iMac.
Basically, if a server is only serving one computer it's much simpler to not have the server at all and just but everything on the main computer.
Basically, if a server is only serving one computer it's much simpler to not have the server at all and just but everything on the main computer.