The 300 hp is only if you get the eco-boost car. It's a turbo charged mustang that might not be for sale. They are still debating on it. I highly doubt they are going to sell a 300hp turbo mustang for anything less then 24k.
I would say if you can get your hands on a Cobra engine with the s/c I would do that. I would say a stock PI engine can handle 500 hp. So a forged bottom end? I dont know if it is worth it. If I were you I would run the stock engine until it dies.