Gonna build an AR platform, questions/advice

If you are completely new to the AR-15 platform, then definitely buy a complete upper. While it is not a difficult build, there are one or two specially tools you will need to get it right.
As to the AR-15 vs the AR-10 (that is 5.56 vs 7.62 NATO), both upper and lower are different, everything is different when you go to .308-okay, not sights necessarily.
Why not just buy a complete rife for your first one--having said that, I have about a half dozen AR-15 and have never bought a complete lower.
Remember, the black rifle is like Barbie for men, one is never enough.

They are very easy to put together. You will need two special tools. One is an armorers wrech, DPMS version is about $30. The other is the upper receiver holder. There are two types, one is a clam shell and the other is a block that is clamped to the vice then the upper is just pinned to the block. I used the second version, about $45 to $50. You also need a torque wrench to do this right. So you will have $100 in tools before you build. If you never build another AR you can easily sell these tools. But be careful because it is habit forming!

Go to http://www.bravocompanyusa.com/ and take a look through their offerings.

I went to http://www.ar15.com/forums/t_3_4/226782_.html to get step by step instructions.

Assembling the rifle from scratch, if you have all the parts ready to go and you have the tools, will take you less than an hour. I easily spent 20 times that in homework and research.

If you really want a .30 class rifle take a look at the 300 BLK. All the power of an AK and an amazing ability to shoot sub sonic out to very long distances. And it fits on a .223 type lower with no mods.

Also be prepared to spend about a grand. You can build one for less, someone will chime in that $600 is possible. You can also spend a lot more.

Just get a spreadsheet, go to bmc and build one on paper. Then add it up. It gets pricey very quickly.

But it is a lot of fun and a great experience. You can space the purchase out over time and buy according to your budget.