1) These machines aren't really suitable for carving .stl files in aluminum. They are just not rigid enough.
Also keep in mind that these machines are not very fast, and 3D machining of .stl files can take a very long time. To get a finished product off of the machine, you need to use very small tools, and make multiple passes very close together. It's not uncommon for 3D carvings to take 10-20 hours, depending on the size of the product, and the level of detail required.
2) What is the largest parts you want to make? this will have a large effect on the answer.
Also, I'm not sure you can expect to see a finished quality aluminum carving straight from the machine. They will likely have a lot of tool marks and require a lot of hand cleanup, if you want it to look like a "polished" part.
As for software for creating toolpaths from .stl files, look at Cut3D, MeshCAM, and DeskProto.