If it were me, I'd like to see a kit for the stock engine that would be basic bare bones, weekend install, ready to go kit. Then I would like to see another section that would provide upgrades that a "budget minded" mechanic/weekend enthusiest could buy and upgrade as time and money pemits.
I myself find it hard to spend loads of money all at once for 1 huge kit. (family and other priorities) It's easier to budget a certain amount of extra money every so often, to upgrade/build on the 1st basic kit, and buy more HP in steps.
A "how much HP" boost could be documented to known stock baseline engines for each part/kit/addition. But, with this said, not everyone would buy all the same additions in the same order, thats the reason I say extra HP over the basic bare bones kit.