What signs of disease are the fish you have left showing? Its hard to suggest a treatment not knowing what is going on. Hyposalinity is free and gets stuff like Ich.
As for the QT ... I would go with the larger 30g if you have a choice. Gives the fish more room and larger tanks tend to be more stable. I'm not sure about brands, but I would get a HOB
filter rated for that size, a small power head just to keep the water moving and probably a heater if you are in an area where it's needed. Also get some PVC
(connectors and L's) that the fish can hide in. That's my basic setup (35g) and it works for me.