1. Refurbs from Apple are every bit as good as new if not better. Apple does a rigorous check on them before reselling them.
2. Apple does have financing available and you can check it out
here. I can not tell you how it works or what their rates are.
3. I own both a 20.5" & 27" and can tell you that there is a considerable difference in size. So if you even think you do not have the room for a 27", go with the 21". There is a huge difference between a 5400 & fusion. I have fusion drives & SSD's in all my Macs and would not consider anything less than a fusion drive these days. It is night & day difference in speed. As for 1080 or 5K, save your money. There is a difference, but unless you are a photographer or editing high end, there is no need for 5K.