The "Machine Controller" is code for a PC with the Mach software loaded. Again, if your even somewhat computer savy you can save here with very low risk. (-$715) - the Mach license is include with the mill so if you have a surplus older windows machine no need to buy..
My hobby has become building CNC machines- I've done 8 or 10 on my own, and lead the classes at the CNC workshop for 22 guys to build machines, so I am really into do it myself.
However, one of the most important features of Tormach is their support, and they offer a unique version of Mach which they customize a lot. In my view it would be foolish to give up all the Tormach support and special features to save a few $ on a cheap PC. The Tormach PC was very carefully chosen to be a match for the mill.
In 'full disclosure' I do support work for Mach wizards, so I might have a tiny gain if you did buy Mach direct and not the Tormach version. I still encourage you to get the Tormach PC and software.