I essentially did the same thing. What worked for me was to pick up the pricey stuff first, then as it was purchansed I could accelerate the build as the parts I needed got less and less expensive. Personally I'd start with a matched upper and lower, they feel so much better when they fit nicely together. Not a functional issue, but if you've used solid guns, bolt actions especially, it can be disconcerting when there is some slop there. Next, to have something to do and push you to go further, get your lower parts. You get those functioning and you just HAVE to keep going at that point. From there, as previously stated, barrel and BCG. Then stock, handguard, and things get cheaper from there (unless, like me, you're building a full ambi gun). At this point you have something that looks like a gun, and the grip, charging handle, and gas block are in order. That leaves your gas tube (if you're going direct impingement and not piston), a muzzle device, some sights, and some ammo. Surely I've missed something (memebers fill in the blanks here) but you're pretty close to blowing money on ammo at that point.
If I were to have any "sage" words of unsolicited advice, buy the best quality everything that you can afford. I found it best to build it the way I wanted it, not the way someone might think I did. Therefore, all the "upgrades" are the original parts themselves. I saw no sense in buying basic anything knowing I'd probably uprade it later anyway. You don't have to buy all Larue/DD/Noveske/LWRC stuff to have a quality piece either. Those are ALL great makers, and any parts you might get from them will be premium and of the highest quality made, you just don't have to break the bank to get really good parts is all I'm saying. Hey, if money is no object I'm going F'n NUTS with the build, but it was for me, so for considerably less $$$ than any of the those listed I think I built something of near their quality and is a piston gun. If you want a build list of what's in mine let me know and I'll post it and you will be able to see where I saved money but didn't give up much.
On that note welcome and good luck with the build.