This will be the short answer since I'm at work and need to be running my MDLs, like I have for the last 4 days.
Short answer:
BWHAHAHAHAHAHAAHAH!!!!
(wipes away tears)
Informative answer:
As I am sure someone else on here has said that designing a kit isn't the issue. The difficult bits are standardizing the kit to fit available and affordable parts, finding vendors that will wholesale at your order size (some have a minimum order number for wholesale pricing), purchasing sufficient quantities of parts without selling a kidney, pricing your kit so that is affordable yet you make a little over cost, generating photos and text then publishing the destructions, purchasing the properly sized 4 mil bag to hold all the pieces of the kit, producing header cards and artwork, beta testing your kit (finding people to do it is easy but dealing with/correcting issues that they find is a challenge) and marketing.
Be prepared for the crushing disappointment that strikes when you realize that the business is sucking the joy from the hobby.
More later....