You can't go wrong with the SPF and the Roush 427R. Since you're buying a pre-owned car, the supply of the market and the cars that you whittle your list down to is going to drive your decision regarding the engine.
Asking prices are going to be all over the board. You're best bet is to compare prices for SPF cars that are listed here on CC, Cobra Country
http://www.cobracountry.com/home2.html, and eBay to see if the seller has set a realistic asking price to use as starting point to negotiate.