With all due respect, I think you've got the cart in front of the horse. To "build" a tac rifle means more than just dropping the factory action into an AM stock. "Looks" can be deceiving. The stocks you've mentioned are really professional grade and high dollar, and for a newbee, kind of a waste of money.
My advice is to enter the game with an entry level package rifle and then upgrade as your skill and training develop.
Your choices in the SPS line really only differ in barrel length. The MSRP you quoted is way high and SPS -Tac's can be purchased right now in the mid $500 range.
You can "build" a reasonably priced SPS-T for a very reasonable price. The rifle will be a regular .5 MOA performer out to extended long range if you do your part. You don't need an AICS to do this on a range rifle.
Here's my "build" an itemized investment:
Stock SPS T .308, 1:12 twist - $565
B&C A2 stock (skim bedded) - $220
Old style Rem trigger group - $0
AZ precision bolt knob - $75
Wyatt DBM - $240 (comes with one 5 round mag)
Harris 6-9 swivel bipod - $90
Burris xtr rings - $54
EGW 20 MOA picatinny base - $40
WoTac 4X14X50 scope - $329
Add various shipping charges and we come in @ less than $1700. We've got an excellent performing entire system here for less than one Nightforce tactical optic.
100 yds., 5 shot string, 168 SMK FGMM:
There are entry level package rifles available through other venues, some well under 1K. PM me and I will point you in the right direction, as it is against the rules here to provide you a link.
Just a little experienced advice. You know what you want and prefer, but, I would at least look at some other options before writing the big checks.