Well hello there my newfound non-poor friend.
For the 11.5 you have two options in the SR15 CQB and SR16 uppers. Unless you can find either in stock anywhere and/or not at 150% retail, the best place I've found where they come up regulary is in the ARF EE. Apparently a lot of people buy them, post pictures of their KAC gun to get some sort of street cred,stare at them for a few months and then can't get themselves to actually take it outside and shoot it because its so sexy and expensive. By that time they've already rationalized that they could get 13 PSA kits and an entire shipping container of Hexmags for the same price as their KAC upper.
If you know what to look for and what questions to ask, I wouldn't shy away from ones that even had <500 rounds on it because you'll never notice the difference.
It's a huge bonus if it has a MAMS on it; if not, get one. It turns the 5.56 into 22LR recoil and allows you to put a KAC suppressor on it as well as any carbine you have that you put a MAMS on.
My 11.5 is a full KAC gun, Magpul STR stock, HK V2 grip, KAC SR25 riser, EoTech XPs3. The riser (hard to find, can only find it in the secondary market) is the perfect height for a CQB gun to be able to bring the EoTech up into your line of sight from a low ready much more naturally as well as allows the use of nightvision without running the NV unit into the rear of the rail. Either way for the CQB, although I don't love EoTech, I'd suggest an Eotech + riser setup for it.
For a SR16 I'd email around and see if they have them in stock or can get them from a few places. There are some on ARF but they are way overpriced for the most part.
KAC Knight’s Armament Company SR-16 CQB Gov’t spec Upper Receiver Group Orders will ship in A WEEK OR TWO! Part Number: KM32480 For the first time Knigh...
nsdefense.us
For the SR25, you don't want a super long barrel on it as ballistically you've hit the diminishing returns threshold way before then, and the gun would be a boat anchor from the front. While I love the KAC 5.56 suppressor, I've always procrastinated on the 7.62 version for my SR25s because of the fact they are not hearing safe and then add forward weight. The rule of thrmub with KAC guns, especially the KAC large frames is that you don't add any non-KAC parts to it because you never know if it'll work due to the variants of AR10/large frame parts.
For a SR25 I'd look at this:
https://lockedloaded.com/product/kac-sr-25-apc-16-med-urx4-mlok#