Being that you are in a second floor bedroom, a 55 might be a little big, but if that is truly the size you want, keep checking for a nice used setup that includes everything. With the possible changes to your move in date, you will have some extra time to wait for the right tank.
I would go with a 29 gallon if it were me. That size of tank will fit nicely in a bedroom and is still big enough for your fish to upgrade......they will love the extra length of a 29. You said you can use your exisitng heater on a 29, so you would just need a lid (glass ones run about $20 at the store), a filter, and a light.
I have a brand new Aqueon quiet flow 30 filter that came with my planted tank kit. I have no need for it, and it is just sitting there collecting dust. I will be happy to send it to you if you want it. It will work perfectly with a 29g from the petco sale.
What did the fish say when he swam into a wall? "Dam!"