Start with a kit. It's fun and you'll understand exactly how and why it works. PSA is tough to beat. I put together my first kit from Delton which is cheap and I've not had an issue with it at all. Depending on what you want, you can be buying some of the parts twice which also adds up quickly. If you get a cheap kit but want precision you'll start off replacing the barrel, handgaurd, and trigger at the very least. These parts by themselves can be more than an entire rifle. However, buying the parts once and not throwing away new parts to replace with better may save a bit.
I'm contemplating a new upper as we speak. I thought I had it figured out, so bought an upper with a reputation for being tight. I was about to buy the barrel and started reading too much and am now stuck between the convenience of factory available ammo and performance you have to load for. I was about to get a .30 American, had it picked out and in the cart, while waiting to go get my wallet I started reading about on game performance of the 6.8 SPC and am now leaning that way. If I do a 6.8 in 16" or 18", I'll be doing a .30 HRT in a pistol.
Also, in case you didn't pick up on it, once you start it's a lot like Meth. You'll constantly be looking for the next little rush.
