If you decide on goldfish, and it seems you really,
really want one, then I'd say put just one into that tank. 14 gl isn't really enough for two fish, but one would live in it quite comfortable, and gets big fast with so much water.
As for a filter, Aquaclears are really great, easy to maintain, and are not that expensive. Here is an example of one...
Aquaclear Mini/20That same filter would of course work for tropicals as well.