If the heater wattage is low for small aquariums it should be ok as long as it doesnt contact the sides of the container. Heaters usually have suction cups to keep them still. As for filtration, I used home made sponge filters (air powered). They make some really nice sponge filters now days that are fairly cheap. A sponge filter that is aged with a good bacteria colony in it can be used to start new tanks and as a rescue from high ammonia in other tanks by just popping it into the tank to help over come the ammonia. Can you see the many uses for a quarantine tank with a sponge filter?? Every one should have one stuck in a corner some where.
When you make water changes , save the old water to fill your quarentine tank if the water isn't too bad. It will cycle the quarentine tank and filter very fast. Do not put gravel in a quarentine tank! A few small stones and a weighted plastic plant will give a fish a little security.