You have a 2005 so you are in better shape then if 2003.
I know many people that never had any problems with their later years 6.0  I would keep it and do what it needs if any thing. They are very nice looking and riding trucks. The cost of new plus sales tax would be breaking point for me.Plus they will tell you that they can't sell the Ford if they take it on trade just to keep value down.
I just bought a e99 f350 drw for 4000 dollars because of transmission problems the owner had. No one wanted to give him much for it as it was and the cost for new trans instlled was over 4500. I bought the truck and found loose feed bolts in bottom of trans. Cost me some new fluid and few hours time 
