For the OP, congratulations. I just finished my first build myself, and it really feels great to have the rifle my way, exactly how I wanted it.
I'm sure you have done umpteen hundred hours of research already, I know I had. I can tell simply because you already named the two companies I consider the best bang for the buck without losing any quality, spikes and PSA. I myself had my sights set on Spikes (pardon the pun) but could NEVER find them in stock. That's when I opened my eyes a bit more and found PSA. PSA is now what Spikes was a year ago. Spikes raised their prices a bit since they blew up in popularity, or so I'm told, and PSA is now the new guy experiencing growing pains for the exact same reason.... High dollar quality at crazy prices.
I'm going to ramble a bit, all the information I spent countless nights researching is all bouncing around in my head, and I'm sure you know most of it already. If you happen to learn a thing or two, fantastic, that makes my research that much more valuable because I helped someone else save some time/frustration.
Don't pay extra for "Cold Hammer Forged" barrels. They DO offer a small increase in the projected lifespan of a barrel. Small. They also cost LESS than the older style barrels to manufacture. Why would they cost more, then? The machine to make them is something like a $20 million dollar investment, so initially they are charging more for them to make up the cost. When I bought my rifle kit from PSA (more on that later) I could have paid an extra $100 for a CHF barrel... Which is somewhere in the range of a 25-40% increase in the barrel cost for what amounts to being able to drive a car 500,000 miles instead of 450,000. The extra cost just isn't worth it, and in however many years it takes for your barrel to wear out (which will be a long time to begin with since PSA and Spikes make their barrels out of 4150 steel and chrome lines them), well by that time CHF will be standard and not cost any extra. Get one then.
Get a mid-length has system unless you are making a short-barreled rifle complete with the tax stamp. Mid-lengths have MANY advantages, most I can't remember, but some are they run smoother than carbine length. It has to do with the dwell time and pressures that come from keeping the gas port 7.5" from the end of the barrel instead of 9.5" like the carbine does. 9.5" creates the potential for quite a few issues, I believe.
Anyways, I got tired of waiting on a ST upper, I wanted to get the ball rolling! PSA had their items in stock and after doing my research, I found that they had the exact same spec sheet as spikes, or even bravo company and other high dollar manufacturers. HP/MP testing, 4150 steel, they make FANTASTIC rifles.
I ordered the 16" mid-length 1:7 twist chrome lined rifle kit for damn cheap, straight off their website. I customized it at checkout like you can do and opted for the MOE stock, MOE grip, Magpul MBUS front and rear sights, and the MI T15 handguard with a low profile gas block all from the factory. Having the complete upper sent to you takes a LOT of hassle out of building the rifle, now you don't need to worry about a torque wrench for the barrel nut or messing with the gas system.
They have recently expanded their handguard selection and include the MI T12 rail as well as the T15, but I prefer the 15". My avatar is a picture of the rifle with the rail on it, I have never been a fan of the fixed FSP. YMMV
As far as tools, I built mine today with a pair of needlenose pliers, a hammer, and a set of roll pin punches. Would I recommend it? HELL NO!
Get a vise, and a lower receiver block. With a premade upper you don't need an upper receiver block, but if you buy one for future. Just make sure to get one with an insert that protects and supports the INSIDE of the chamber for the install of the barrel and handguard. Roll pin punches are a must, and roll pin holders would have made life simple. Be warned, I spent an hour on the floor of my kitchen searching for my rear takedown pin detent spring after it shot across the room. I found it under my stove. Those things are a bitch and a half, especially the rear when installing the magpul ASAP plate because of the two ridges that hug the lower receiver. Make sure you are in an area where you can find lost springs, just in case. You might want to order a LPK spare parts kit just in case, they are always good to have around and are cheap, just the springs and detents or so.
What else, what else.... I already THOROUGHLY endorsed PSA and Spikes, with preference to PSA. Talked about tools, CHF barrels, and gas systems.... The rest is really up to you, it's your build. I think Magpul could **** gold, and anything they make is good to go in my book. Unless you are going to do competition sharpshooting, go with a chrome lined barrel. A regular A2 flash hider will work fine unless you are a night-vision using badass, and a compensator will help with recoil but that's generally saved for guys looking to shave every millisecond off their shots for competition.... Plus anyone at a range next to you will hate you. It's a .223, a compensator isn't needed unless your a competitor (or a wimp
Oh, optics. I opted to go with a bipod and vortex scope for fun. My rifle will be mostly for target shooting, and vortex makes great glass all the way down to their $200 scopes. Primary arms makes a fantastic micro dot for $80 if you are against shelling out $500 for an aimpoint or a EOTech. Primary arms also makes great magnifiers if you want to magnify your red dot or whatnot, look in to them. Optics are different than the gun itself, and it is truly a game of you get what you pay for, but Primary arms is another example of a company making a product (it's micro red dot) that costs less than its comparable counterparts.
Have fun with it, above all.
Attached is my first baby I put together today
ETA: schpelling errurs.