I had a look at NYC CNC’s website and while it might be a good resource it seems to be geared towards industrial level machines. The kind of machines that don’t even list a price on their website, requiring you to request a quote instead.
Since I don’t consider $50,000 machines to be budget friendly I would suggest having a look at the resource pages for the LongMill. While those pages are about Sienci’s LongMill MK2 the first two sections, Basics and Software, should provide good information for someone who is brand new to the CNC world.
I think Sienci’s AltMill would be the better machine for aluminum, but I’m no expert on milling metal, the resource pages on the AltMill are here.
That should get you started and then maybe you’ll have a better idea of what to search for next.
EDIT: I wanted to add that your use case will impact what kind of machine you need. You mention aluminum molds and I think if your objective is to make a mold and then sell things made with the mold a hobby level machine is an acceptable way to get started. If your trying to make a ton of molds and sell them then your machine requirements are much higher.