Mick Neil says it all really but straight off the bat I can tell your design is probably working along the wrong lines for good machine because you mention C beams. This type of beam isn't often used because of it's weak flexy nature.
This is why we ask new builders to start a thread and give some idea of the design. Even if it's just picture of fag packet sketch.
Like any structure you start at the foundations and if these are flawed then the whole structure fails. The Machine design then leads you to what components are required.
Building a good CNC machine is done in orderly stages and nearly all those that shoot off buying components before the design is finalized waste money.
Start a Build thread mate.