From my own experience, I can tell you that you are going to be hard-pressed to build an upper for cheaper than you can buy them already assembled. If you just have to have it a certain way (this bbl v. that bbl) contract with a smith to assemble it for you. Otherwise, you're going to have to buy some tools that you won't need very often.
On the lower, building it yourself is the best option because it usually does work out cheaper. I chose Stag and couldn't be happier. The quality of their LPK is head and shoulders above what I've seen from other makers, and the lower itself is a beaut!
The Vltor stock, with the battery tubes in the cheekpad, is nice, but do you really need that storage space? Think about it before you pay a premium for something you really won't need (this goes for all the stuff). Unless you're living in the field and resupply is sporadic at best.......
There are so many ways you can go, it's actually pretty funny. I just wanted a simple M4gery, and now that I have it, I'm thinking about what I want on the next rifle. It just doesn't end!