Just buy the land and ignore the set back requirements. You aren't building anything new so you don't need a permit. The structures already exist and you aren't changing them. Your pole barn might technically be in violation of the setback rule but it is a preexisting structure. If the setback violation doesn't bother you and it doesn't bother the neighbor then nobody cares. You might run into an issue down the road if your neighbor sells her land and/or if you decide you want to upgrade the pole barn. But if you just draw a new property boundary and leave all existing structures as is then I don't see any reason why you need a variance. You aren't doing anything so you don't need permission to do something.