I'd buy the upper and put it on your lower if money is tight. They are an exact fit and an upper isn't that much, maybe just a little more than the .556
Came across these guys the other day. Their prices are, well, cheap. Not sure about quality. You could probably get some info on them over on 68forums.com.
I also suggest building what you want. The ready to shoot guns are a lot more $$$ for really no reason. My 16" will shoot about anything well, my 21" likes heavier bullets. If you're going to hunt with it and use anything other than TTSX you're doing yourself a disservice [emoji6]
I hunt with a 16" 6.8 Stag upper. Previously used a 20" bull barrel stag model 7 upper, but it was much heavier. Stag has there complete uppers on sale for black Friday. https://www.stagarms.com/black-friday-d ... er=6.8+SPC