A site is not a residence and therefore not relevant to the FTB status for stamp duty.
The value of the gift of land from your aunt, is the value at the date of the gift. If the land, at that time, does not have planning permission, it will be valued at the agricultural value. If you do succeed in getting pp (and there are no guarantees) then it will be your efforts that will have increased the value. If you don't get it, you are stuck with a field that isn't worth much.
It would be advisable to get an independent valuation of the land at the date of the gift, in case of any queries from the Revenue.