If you get a Refer that is used and you do not know how long it has been sitting, you could be asking for it. The reason being, the chemicals sit and when you turn it on and less than 30 days, they eat a hole in the pipes (cooling unit). Then you have nothing. Also, having a Refer that is a lot smaller than the one that was in there, you have to make sure that the inside is completly sealed. Carbon monoxide can take you away from here.
If you do seal it enough to be safe, it can look tacky. Saving and getting the model that fits in the hole is the best way to go. Do not get the 3 way, unless it is the only way you can get it. It will cost you more and is a maintain only feature.