If it's not badly-damaged, you may be able to repair the rim instead (or have it repaired by someone). If it's just dented or slightly out of shape, a firm hit with a heavy hammer and a round-end punch could put it back in shape. Put some paint on the scratch and off you go. Have it checked/balanced by a tire shop; if it does not come back with an excessive quantity of weights, you're fine. Usually, the shop will tell you beforehand if something's wrong.
If you notice a crack on the wheel, junk it. Give it to someone that could re-use it as scrap metal.
If you end up buying a new rim, since it's at the rear and your car is a FWD, you should not have to worry too much about its size or its weight. If it were on the front, a significant size difference can damage the differential on the long run, and a significant weight difference could affect acceleration and deceleration due to the different inertia of the wheels. Just make sure that the size with new tire is close to the wheel on the other side.