The 53K has been one of my favorites. I had them built with SW53 AR receivers so I can use Beta C or Surefire 100 round mags. Not quite the 51K but 1/2 the money to shoot. If I want a little more challenge, I have a couple of SW MP5K - 10 clones. Being MP5K sized and speed but in 10 mm AUTO it is definitely a handful. I see several have listed the MP5 SD. I personally perfer the MP5-40 SD. The standard MP5 SD in 9mm is some what under powered on steel. The 155 or 165 grain .40 S&W has a lot more knock down power than the 115 or 124 grain 9mm SD. And it is pretty quiet. There just have been more configurations of roller locked made than for any other platform. Eventually I'd like to have one of my two .40 S&W SDs converted to 10 mm. And I still want a MP5K clone in .45 ACP.
The oddball hosts aren't cheap, but anything roller locked is going to be pricey. Now there are those that get a full si MP5 model, a 5.56X45, and maybe a 7.62x39 clone because it is cheap to buy and shoot and call it a day. There is also those that must have their hosts with a HK receiver. My brother bought a shooter grade 93, a pristine 53 parts set, and had TSC Machine build it. He had $6,000 in the build. It is a beautiful build. But for less than $6,000 during Todd's "going out of roller locked" sale I bought a full size CA MP5-40, a CA 9mm reverse stretch, a CA SD with SS can, a CA53 AR, and two Vector .40 SD parts sets with .40 caliber SD cans. I think that I got the better deal but my brother wants HK receivers in his sear hosts. As many have said, there is something about a belt fed. YMMV.
Scott