Yes, you can register after you get to the resort. When you add the vehicle to use transponder it will be registered as a rental and will also have beginning and ending date, so no you do not have to un-register.
You can aslo set up to how to add dollars. I add dollars once mine drops to 10.00. One thing, the dollars never expire so they'll be good for you next trip.
I think it's worth getting the transponder. ($25.00) I used to take a bunch of change, now I don't, and you don't have to stop to pay. I don't like getting in the pay line and the person in front of me takes forever wanting receipt, change, directions or they only have one lane open and your in line for 10 min.
You can order by mail and have it shipped to you. They did not charge me and I had it activated and ready to go once I arrived. No problem going through security either and it's in my purse. Last time I forgot to register rental car and had no problem. I only used it though to go to airport & back on that trip.
Also, if your rental car has sunpass option, your transponder overrides it so you are not charged by the rental company.