If it's a female you may not have to move it at all. If it's a male I would probably move it now and keep something smaller or just plan your renovation around your turtle rescue.

The alligator with proper care will be 6-8ft in no time and it may not be the best use of funds.

However building a custom aquatic area is possible either with concrete or out of wood that has been properly sealed. A third option is to use one of the larger stock tanks with a deck around it. There is a 10 x 10ft stock tank available that would be a sufficient water area until the animal reached a large size.
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CDieter
'Reason, observation, and experience; the holy trinity of science.'