I have been out of the country for the better part of 8 years for work (military, but Air Force, so my rifle practice hasn't been kept up to snuff), and am finally coming home! I used to be really into competition shooting, and was trying to get into 3 gun, but then got told I was moving to England. I sold my AR-15 before I left for a crazy good price, so naturally I need a new one. My dad has all the tools and everything to build it, but I haven't kept up with new brands and who has the good stuff.

Planning to build a competition gun with a freefloat barrel 16-18 inch. I already have access to an Aero Percision lower from a build my dad changed his mind on, but haven't bought it yet and don't have to. I have budgeted about $1500 to spend (amazing how much money you save when a gun guy can't buy guns).

Also I think it would be cool to use a piston system, but I haven't been able to find many kits or info about them.

So please, any suggestions about brands or specific parts or anything, even new part or niche equipment (like the bad lever or something) that I should look into, would be greatly appreciated.

You can build an excellent rifle with that budget. These parts are what I frequently use.
Complete Uppers: Bravo Company
Bravo Company also has complete lowers for $400, bigly excellent choice, comes with a good FCG, for mil-spec that is
Receiver, upper & lower stripped: Aero Precision- ArmorAlly is my first go to place, shop around!
Most Important Part- Barrel: Faxon makes excellent barrels at a reasonable price, but don't go cheap on a barrel. Colt, BCM, Noveske, DD and Larue (probably the best since they are made in TX )
Muzzle device: standard or extended A2 hard to beat $10
Gas Block: Faxon also has, I am partial to BCM and Geissele. Some barrels come with GB's, Geissele non-blem rails come with GB
Rail: Geissele (especially when on sale, especially blems!). Midwest Industries and BCM are excellent choices
BCG: Shark Arms has Colt for $125; Toolcraft is my next go to BCG ~$60-80, ArmorAlly my first go to place for Toolcraft. Note- Toolcraft only makes carrier, bolts are outsourced so check specs, ArmorAlly is gtg source
FCG: Larue MBT $87.
Buffer: I find a carbine buffer laying around and make a specific weight buffer using tungsten weights from here, using "5OFF" coupon
Receiver extension- 7075, carbine buffer, action spring kit: Aero Precision, shop around, I've gotten mine for $30-40
Action Spring: I only use Tubb ~$30

Check out www.jprifles.com for parts or a whole AR.

Buy a LaRue Ultimate Upper kit and a lower receiver of your choice.  Done.