How much money do you want to spend and what are you looking to do with a 53K SBR? I had Jeff build me a couple of 53Ks using SW53 AR receivers. Since I have a sear, he set the 53Ks up as pistols. I also had Jeff build an A2 stock using a green stock. I also had a surplus G3 A3 stock that I had Jeff modify for use with the 53Ks. The barrel of the 53K is thicker than a 9mm "K" barrel. So I bought a SW "K" grip back when there was Black Market Parts and opened up the center of the grip for the wider barrel. I bought that "K" grip specifically for my 53Ks. My guns and A3 stock are black. I'm sure that you can sear Jeff "Dragons" thread on his forum.
As I am sure, if you have one of Jeff's 53Ks, you are aware that the 53Ks that Jeff builds uses a custom collapsible recoil assembly that is welded to the end cap of the converted G3 end cap. It is my understanding that since the 5.56X45 in a 5.1" will have almost twice the speed of pistol caliber. So there will be almost twice the friction heat in the 5.56X45 "K" barrel than a 9mm "K" barrel. The plastic "K" grip does not have an aluminum heat shield that other rifle caliber hand guards have. I would highly recommend wearing gloves when you shoot it. With a standard factory flash hider I usually get a two foot diameter flash ball when I fire my guns. If I pull the flash hider off, it becomes closer to a three foot diameter fire ball at the muzzle. Even in bright daylight after the first round is fired off, If I look at the sights I can't really see much for a sight picture, just "flash".
To me, I think of a 53K as an expensive noise maker. I usually insert a mag, give it the HK slap, point it in a safe direction, close my eyes and pull the trigger. To which I usually receive a couple of hoots and hollers from the guys I'm shooting with. The gun stops, I open my eyes to cock back and check the chamber. With a black gun and a black interior of that gun, if I don't close my eyes I can't check the chamber very well. I have run drills, during the day with my 53s. I have never tried to run drills with my 53Ks. Maybe with a Vortex flash hider you might be able to get the flash under control enough to see the sights in the daylight. That seems like a tall order. But I have to admit I haven't tried to. I would highly recommend that you shoot your pistol a bunch to be sure that you can see your sights before spending money on a tax stamp, engraving. I sincerely wish you luck with 53K SBR project. Maybe someone that has done the SBR process with their 53K can speak to the flash issue.
Scott