Hi,
it was not recommended to run an exit on an connection that's used for private traffic. It could be your IP that gets caught downloading copyrighted material. (Might not apply in your case)
Should you still consider it, please read: https://blog.torproject.org/running-exit-node
Now, what about the uptime issue ? Is it worth (again in a Tor network perspective) installing a bridge on a computer which is up only a few hours a day ?
Yes. I got told by an Tor official that such a bridge is useful. The longer the better, but a few hours is OK.
Regards, bastik_tor