Don't even talk about a cage until you have learned to weld and have several projects under your belt. Keep reading about cage design, there is a good thread in the Fabrication shop "stickies".
Welding is a lot of fun, but it's also a lot of practice. I have been doing it for a few years now and I'm just to the point where I would even consider doing a cage (which is coming soon).
Personally, I would keep saving and go with something like the AtoZ sport cage additions. I have a buddy with a bender, so I'll probably be making my own based off of that.