I built one, and with care, was able to cut aluminum with it. A couple others here too like Crane has cut aluminum with his. The nice thing is, once you have one built, you cna make better parts for your machine, using your machine!
I would stick to the plans. Once you build it and see what it can do and what its limitations are, you'll have a better idea of what you need to get to make your machine perform the way you need it. Heck make all the parts out of solid aluminum!