Don't worry about the little things right now.
Focus on getting your stripped lower, and your parts kit first. Your ambi-selector will be with the parts kit.
Ambi parts kits are about $80.
Currently, stripped lowers are about $100. Pretty much all of them are close to the same quality. I like Stag personally.
Oh yeah, and some tips:
Don't get a huge charging handle release. It stabs you in the side / chest if you have your weapon slung in front of you.
Don't get a carry-handle AR for your first. Everybody always gets bitten by the "optics" bug, and carry-handles are a pita to work around.
DO invest in a GREAT bolt-carrier group. Cheaping out on it, will only hurt you in the long run. Daniel Defense gets my money.
If you're assembling your own AR upper, USE THE PROPER VICEBLOCKS. YOU WILL damage your upper if you don't. Don't ask me how I know this. :P
DO join ar15.com.
GI 30rd mags are just as good, if not, better in some cases than polymer mags.
Most importantly, make your AR how YOU want it. Some of us like VFGs, green lasers, flashlights, red dots, and magnifiers. Some of us like K.I.S.S. carbines. Some of us like retro-builds. Don't let someone MAKE you do something. Make it how YOU want it.