I love the idea of building something from scratch and have even tried doing this on a variety of ideas over the years. One thing that I've learned the expensive, and time consuming way, is that if you can buy it already built, regardless of the cost, you are saving money.
Everything that is already built has gone through a trial and error process. No matter how much thought you put into your design, the unknown will show up and point out what you need to redo, or scrap and do differently.
A proven, time tested product is sold for a profit, but when figuring in what it took to get it there, and what it costs to buy the materials and time to make it, you will be hard pressed to do the same for less on your own. Also keep in mind that nobody is getting rich off of one item. They have to sell a lot of them to make enough to make a living, and even then, profit levels are not all that great. Just try to figure out what the price to make it will be in materials alone and you'll find it hard to compete with a name brand supplier.
An excellent example is with trailers. If you try to build your own trailer, you will spend more in parts then you can buy one already built. You have to buy huge quantities of materials to get the discounts on materials that the trailer builders get, and then have welders who have dont it so often that they can cut and weld up a trailer in the shortest amount of man hours. Until you get to that level, you cannot compete with what the manufacturers can sell it for by doing it yourself.
Same thing applies to mills and impliments and just about everything out there.
Eddie