You my very well run a relay, given your limitations. A Tor exit
may be a hassle, you should do it only if you are willing to accept inquiries by your ISP, angry people claiming you are spamming them, and so on.
As for the difference between a relay and a bridge, it is entirely up to you. The amount of bandwidth you are willing to dedicate to the service may be passed on to the Tor configuration file,
/etc/tor/torrc, which in fact states:
Use these to restrict the maximum traffic per day, week, or month.
Note that this threshold applies separately to sent and received bytes,
not to their sum: setting "4 GB" may allow up to 8 GB total before
Set a maximum of 4 gigabytes each way per period.
AccountingMax 4 GB
Each period starts daily at midnight (AccountingMax is per day)
AccountingStart day 00:00
Each period starts on the 3rd of the month at 15:00 (AccountingMax
is per month)
AccountingStart month 3 15:00
You may thus limit the ttal amount of traffic per month, as you see fit. Tor is quite elastic about this, the amount you have to donate is perfectly fine.