A true "K" like my build is something that I will never do again, no point. To get the 3/4" less length you sacrifice a lot in terms of structural rigidity and you loose all the regular MP5 stock options which are plentiful and far less expensive than "K" stuff. The K stock essentially slides into the receiver and thus the need for the reinforcement plates and the two pins, one at the top. Even with the extra pin and the reinforcement plates, I find it far less tight and it still moves around quite a bit, even on real HK made guns.
You are certainly entitled to your opinion, but your experience has not been mine. I find that my HK folding stock locks well and shoulders well. The "K" stuff is more rare. HK parts are pricey and rare HK stuff is even more expensive. That is just the way it is.
The K lowers, the real ones, cost an arm and a leg. I got an original SP89 lower for my build and you don't want to know what it cost. With a reverse stretch you get tons of lower options at half the price. Home made K lowers can really suck bad if not done well.
Like anything, there is suggested retail and what you might find with patient searching. My first "K" build was with a complete HK MP5K parts set and a SW receiver. The complete HK parts set was $1,700. It came with a burst lower. Using that lower, I had a local machine shop modify a couple of G3 "Navy" style SEF grips for use with the "K". I have multiples done as I have many "K" guns in multiple calibers. From personal experience a 10mm "K" is a hand full.
K was designed to fit into a suitcase and 3/4" less length makes no difference in the real world.
I'm fortunate enough to have at least one in "K", "K" PDW, Reverse Stretch, full size and SDs in 9mm configurations. Of all my 9mm roller locks, the "K" is my favorite. I've used that gun in a couple of classes with Teufelshund Tactical. The "K"is more concealable, and faster to deploy than a standard MP5 especially with the HK harness for the "K". I find the "K" is lighter so transitions from target to target is faster and I'm less likely to "swing through" the target with a "K". BTW, it is my understanding that the "K" was developed for conceal ability. The briefcase came afterwards. I'd suggest checking before posting something as "fact".
With the custom sights and finish, why didn't you put the paddle mag release in it? If you don't like it, why don't you sell it and roll the money into something you want? Again, you are certainly entitled to your opinion. My biggest problem with your post is your statements like "K was designed to fit into a suitcase and 3/4" less length makes no difference in the real world." That might be true for you. I for one disagree with blanket statements. In my personal experience that 3/4" makes a big difference. I'm stating my opinion as such. Your statement comes across as statement of fact. That is a disputed "fact" IMHO. YMMV.
Scott