just remember, it's a good idea to solid mount the trans as well if you're going to solid mount the motor. But i wouldn't think it'd be worth it for power gains.
I noticed a difference when i made an intake for mine, as for exhaust, I'm not into the whole loud exhaust, only reason i replaced mine was b/c the old one was falling apart. Then the new oe replacements kept breaking so i have a magnaflow on it now, which has been good, and i thinks it's a little on the long side for my truck, so it has a nice quiet tone, but i didn't notice any power increase.
For plugs i tell everyone stay with autolite or motocraft to avoid problems. And i doubt you'll have any power gains from changing plugs unless the old ones are toast. And unless the wires are bad, i'd leave them be as well.
You can get a stock header style manifold from a 93-94 ranger from a junkyard for next to nothing and that will help.
Lastly, if you can pick up a turbo motor for $200 dollars, do it! and stop wasting your money on petty crap!