It all depends on how you want to approach it.
1. Add a supercharger to stock engine and a intercooler pump it up to 17 Psi get it tuned and chipped add NOS and take bet’s on how long the engine will last. But you will see good power gains, depending on how much potential is in the stock engine.
2. Get the heads ported and polished, bore the intake and exhaust valves out, new valve sprigs and valves possibly a new lumpier cam, Extractors + exhaust new fuel injector’s, race pistons, custom built intake manifold and twin throttle body’s (One for each cylinder bank). Then move on and get it chipped and dyno tuned. (###* yeah)
Option 2 will probably blown the budget a bit!!
Just one question it is a Explorer not a sports car why would you spend all that money on engine Pro-Formance for a 4WD, I think you are driving the wrong car if you need speed and horse power and want to run 10second ¼ miles go and buy a V8 or 6 cylinder spots car which is engineered for speed and handling.
If I had $7000 lying around I would be spending it on off-road modifications. At 155Kw and 340Nm of toque the V6 goes all right and isn’t’ exactly slow.
Good luck with the project regardless. Just talk to ford as well to find out the horsepower rating of the block.