Ds213j As a server

Would the synology ds213j be good as a server?

Probably not.

If you are only using HDHomeRun physical tuners and have no plans to use remote streaming or commercial detection, possibly.

IIRC, that device uses a small underpowered Marvell or similar Arm7 processor, so anything beyond simple file streaming would be a draw. Also, there is no TVE support for that processor.