This question seems to pop up every few months. There is a market for it, but to put it simply, these guns are not cheap to manufacture, yet the cheap American consumer wants a military grade firearm for $600.
Your options here in the US are:
HK94: $4000+, if you can swing it, do it.
Special Weapons, Coharie, Black Market Parts: These are the Todd Bailey guns. Some people are unaware that Todd did not always make all of his own parts. For a while RCM was making parts exclusively for Todd. They, like everyone else, had a falling out and they are no longer affiliated with one another. Todd argues that RCM is the reason for those of his guns that were problematic. RCM disputes it, saying they made all parts to Bailey's specs. Fact is that RCM is now known for making excellent parts for roller-locked HK pattern guns.
Bobcat Weapons and Vector Arms: People erroneously lump these together with Todd bailey's other companies. They are not. They had agreements with Todd to use receivers, barrels and parts (exact lists unknown), but Todd did not bend them, weld them or finish them. Eventually Todd decided he wanted the market all to himself so he broke off his agreements with both these companies.
MKE: These are the Turkish clones previously mentioned. Some folks claim they are the best thing short of an Oberndorf rifle. My opinion is different; the ones I have seen all had some nasty welds and appeared to be made from used parts kits, and were marked in a fashion which supports it. Some folks say the earlier rifles were much better, which may be true. I have seen 5 or 6 of them, and all looked like poorly made kit guns.
Bellator Arms: I don't know much about these other than they are most likely made with the latest Black Market Parts (Todd bailey) barreled receivers. From what I understand the chambers are un-fluted, which is strange. I believe they also have a strange ribbed cocking tube in order to skirt the trade dress issues from H&K.
Getz Firearms: He's a member over at HKPro. I believe he may be using receivers from Todd bailey, which isn't surprising b/c there's very few, if any, people making MP5 receivers right now. End user reports say that these builds are great.
Dakota Tactical: These are custom builds made using RCM parts or German kits. Build quality is reported to be excellent, but there is a significant wait time. Not sure whose receivers they are, most likely LSC flats.
Parabellum Combat Systems: Also known as "Ghillibear," Jeff is a custom builder. Super nice guy who is very talented and capable of producing some very interesting odd projects. You provide him with everything and he builds it.
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Personally, I have a BW5 that has been SBRd and it runs great. It had some problems feeding flat point ammo, but that is a pretty common issue with MP5s; there is no feed ramp to speak of and the rounds get slammed into the top of the chamber in order to feed. The only gripe I have with mine is that the barrel was not pressed in with pinpoint accuracy, so the rear sight needed to be drifted a good bit to get zeroed. I'm a "set and forget" guy so I could care less.
To me, if you've got a good barreled receiver with an RCM bolt group and a genuine HK trigger group, it is a beautiful thing. Most problems can be solved with a $10 extractor spring, a $50 German extractor, or a $50 ejector. A lot of people get upset that they would pay over $1k and have to debug it, but "it is what it is."
Video: The Truth About MP5 Clones
Me...I don't regret it one bit...
In more classic dress: