there really is no answer to this... but the first thing I did with mine is get it back where it was supposed to be from the factory. New plugs, wires, air filter, oil change, check all the fluids, wipers, check your tie rods, brakes, alignment, tires, shocks, struts, steel fuel lines, steel brake lines, and electrical system.
When all that is back to where it was supposed to be BEFORE the stang had a billion miles of abuse then I start thinking about mods. I have a 5.0 so I started with a BBK CAI and 65 mm throttle body. I just picked up the GT40 upper and lower intake manifold and have been playing with the thoughts of GT40 heads. In your case the plenum and TB would be a solid start when paired with a complete exhaust system. Personally, I have BBK shorty headers into BBK H pipe (I like them better) and BBK exhaust. The real power adders for me will take some time, like the heads bc I will have them ported and built, and paired with a TFS cam while im torn down. Then maybe a supercharger at some point...