Really, what you are asking is whether or not you want to be an owner/builder. Buying materials is not ever going to be a concern (which is all those kits are...pre-selected materials that they send to you on a truck). Being an owner/builder is completely doable and can be financially rewarding...but there are plenty of boxes to tick before looking at materials or kits.
To answer your original question, buying a materials kit like you described usually isn't worth it, especially if it's a standard stick framed building and not something special. Invariably, you'll have either too much or not enough of something, the engineering will be wrong for your local building codes, etc, etc. You're MUCH better off purchasing your materials from a local building supply company (not a big box store). They will gladly take your plans and do a take off for you...especially doing exactly the same thing a kit company does. They will either be cheaper or close in price. They will work with you on returns or shortages and usually will let you open an account while you build.
But again...there are like 50 things that should happen prior.
First and foremost...have you lot/land FIRST and design your house to fit the land. You are looking at kits with widely varying footprints. What if you decide your perfect building lot is narrow? Or you have great solar gain and want to take advantage of a passive solar design.
Second, materials costs are like 1/4 the actual cost of building. Does your lot have utilities? Will you have to install a power pole, septic, a road, etc? How much of the work will you do yourself? What about foundation? Mechanicals? Permits? Acting as your own general contractor can save you $$$ but believe me, you'll earn it.
The process is plenty challenging without having to try and deal with a material supplier that's a thousand miles away.
My $.02.....start looking for a building lot or land first. While doing so, research the building process in your area. Regulations and requirements vary widely. Talk to the building department about your potential project. Find out exactly what the engineering requirements are (wind, snow, live/dead load, etc) and what building codes you'll be expected to follow.
No matter where you get your materials you'll be expected to abide by those regulations. It would be a bummer to have a building shell up that's rated for a 40lb snow load only to have an inspector tell you your area requires 100 lb. Build everything on paper first and you'll most like avoid the bigger, nastier surprises.
Good luck.