If you really want to sew, I suggest a getting a serger in addition to a main machine. Trying to deal with the zig-zag stitch on one of those computerized machines sucks.

My cheap-o brother machines have died slow deaths and I've replaced them with Juki. TL-2000qi and a MO-644d

What tools might I need?

not much really. seam ripper, good shears (ginger is a great brand), a long ruler (this guy works great), denim needles.

chalk (I only buy Clover brand)

I prefer binder clips over pins. Don't try to sew through pins with denim - great way to break needles/machines.

I'll probably be buying cone denim, but what do you use and where do you source it?

this is a great source for denim, thread, buttons, and rivets. They even sell a kit with everything including a pattern.

What about thread, buttons and zippers?

above source for all of that

Any advice or pro tips?

serge everything but the selvedge ends. especially if you do stretch denim. makes the whole job about twice as fast.