From the looks of that plug it looks like it has about 200k miles on it, not a good thing. Your oil leak is another matter, you need to face the fact the with about 300k on that engine it is about shot. You spend the money to replace that head gasket I fear you will be throwing good money away. Now if you can afford it, get a manufactured engine and replace the old one. I doubt if that transmission has 300k on it but if just might. A couple years ago I rebuilt a Ford 5.4, it isn't an easy engine to rebuild, I can assure you of that. If you decide to replace it, PM me and I will put you in touch with a company that has top of the line reman engines, and at reasonable prices. Example 5.4 is about 1200 dollars exchange.
Check out the ford forums, you may gain some other insights from them.