I don't normally post and most times I have just been reading and searching.
I am in a similar position as you. I am a SSG/E6 in the National guard and don't get enough trigger time so I am looking at building my own rifle.
If your unit has the M16A4, it is a 20" rifle with a flattop with a removable carry handle
If it's anA2, it has a fixed handle.
In the M4, we have rifles with the removable handle.
With the optics, our unit is using the M68 red-dot. Dont mean anything until you hear that it is the Aimpoint ML2 which is compatible with night vision googles. The big difference between the ML2 and the ML3 that I have found is the water depth the ML3 can go to over the ML2.
The other optic we use on our rifles in the Trijicon TA01NSN 4X32 ACOG (Advanced Combat Optic Gunsight)
For all practicle purposes for now, just get a 20" rifle with a flat-top, carrier handle, gov't profile barrel and practice with it. It is the closest you will get to the rifle you will use in basic. As it is, you will have the badd fortune of shooting a similar rifle that it MAY get you in trouble. A drill sergeant takes his job serious, and part of that is to teach you how to handle the M16. At least when I was in. Anyone that showed any inclination of knowing how to tear a rifle down prior to his instructions, got lots of push ups to do. The hardest part of shooting an M16A2 or A4 is to learn the trigger. It has in effect, 3 sears from the burst mechanism. As a result, I see lots of guys/gals pulling that third shot with a heavy sear. Just an FYI.
As you won't have much to spend your money on while in basic, you can come home then and get the same upper and optics when you get to your unit and have a chance to talk with the supply guys/armorer.