I really like Seekins Precision handguards, Trijicon MRO red dots, and Hiperfire 24c triggers, and Magpul ACS-L Stocks.
As far as brands of rifles to start with, it depends on what you want to do with the rifle. If it's to shoot 20 rounds per year and sit in a safe, most any of them will do. If it's to shoot a couple of thousand rounds per year, still lots of them will work fine, but you'll want to think about things like a chrome lined chamber/barrel/bolt parts, etc. Daniel Defense is hard to beat for durability, but if you want super fine accuracy, you might do something else. I have a 5 year old plus Bushmaster with several thousand rounds through it that still functions fine and accuracy is still good enough for for action rifle matches.
Also, 9mm AR-15's are in style right now and there are lots of really good choices, but I think I'd personally only feel good recommending a Colt. Not to say others aren't good, but I've seen some pretty spotty stuff from other manufacturers.
Unless you're wanting something very special, I don't think I'd recommend building your own rifle. The market is flooded with cheap parts and with cheap parts guns right now. If you want a cheap gun, buy a cheap gun and you'll still be able to sell it if you don't like it. Build a cheap parts gun and they're hard to sell right now. Of course, if you just wait until the next anti-gun president gets back into office, you'll be able to sell anything remotly resembling a black rifle. Good luck.