Don't you mean exhaust manifold?

Stick standard plugs back in as the motorcraft ones are what works best in the zetecs, and the induction kit is probably loosing you power due to the heat soak, so I would use standard air box and a good panel filter.

Thought about throttle bodies? Only problem is anything that you do to it that is going to get good, noticeable gains out of it will require aftermarket management, shich is all going to be way out of budget.

Why not strip the interior out, make it as light as possible, put some ST170 brakes on the front and put some decent suspension on it, as that will make a massive difference to how quick the car is without touching the engine at all.