If this is your first AR15, I would get a basic 5.56 carbine, because they are the cheapest, you can change out parts/enhance as money becomes available, every problem has been documented (so if you have a problem it is easy to troubleshoot) and if you decide that you don't want it you can easily sell it or keep it in the closet.
One of the fun things about AR15s is it is the leggo of rifles, you can swap out pieces parts and major assemblies with little trouble. Since it was designed many moons ago, the design is off patent, so almost all of the manfacturs are making milspec AR15s. Additionall, most parts are cut on a CNC machine, thus interchangeable between manufacturers (but some manufacturers have better quality control than others).
For Ammo, go to Ammoseek.com. Ammoseek is a web browser for ammo and reloading supplies.
Lastly, If you want to get into serious target shooting, it is best to spend the money and get a rifle that has been hand tuned (bolt and chamber mated, gas tube centered, etc... ). Your generic AR15 can easily group 4 MOA with any type of ammo, but to get in target country you need at least 1 MOA with target ammo.
Armslist has just gone pay-per, at least in my area.
The memberships for private individuals aren’t outrageous, $6.00/month for credit-card memberships, $4.00 per month memberships by way of pay-pal & other ACH accounts.
Spose it had to happen but it’s a bummer after all these years.