I like your idea with the cheap oil. When I did mine, I just waited for its oil change time, put in the Seafoam and ran for a few tankfuls, then did the oil change. Either way is fine.
As far as the tune-up goes, all I can say is that it's "cheap insurance." When people talk about oil changes and tuneups after seafoam, they just keep in mind the fact that Seafoam has the potential to knock a lot of crud loose and you don't want to run your truck with that crud circulating in your oil or on your injectors. It all really depends. You're gonna have people on both ends of the spectrum, people who put in Seafoam and do nothing, and people who feel the need to replace everything afterwards. Personally, my truck could use a tune-up, but I just did an oil change so I'm gonna wait till next oil change time, put in a can of Seafoam and run it for awhile, then do a tune-up and oil change then.
I don't think you'll need to change your plugs unless it makes you feel better about it, especially since that could get expensive. And you definitely shouldn't have to every time you Seafoam, just possibly your first one in case there's alot of junk build-up. But, like I said its cheap insurance and peace of mind and since everyone has their own idea of what is right and what is wrong, read about it as much as you can and do what makes you comfortable.