Being that Heckler and Koch is a German Company
Their "Assault" type rifles and Carbines are no longer legal to import into the US for civilian use.
While the 1994 Assault Weapons Ban has expired there are still alot of restrictions on the import of foreign manefactured "assault" type rifles. The HK's fall into the catagory of banned import weapons.
HK's civilian version of the MP5 was a HK94 basically a 16" barreled semi-auto version of the MP5 also chambered in 9 x 19mm. The HK SP89 is the pistol version of the MP5 weapons system, and the HK SP89PDW (Personal Defense Weapon) is the semi-version of the MP5K (Kurtz) The HK51 is actually a .308 caliber carbine, and not considered a part of the MP5 family.
As VUPD mentioned there are several US manefacturers that are producing MP5 "clones" in legal configuations like Bobcat Weapons, Ohio Rapid Fire, and Vetor Arms (though they do not offer a 9MM version). Special Weapons also manefactured MP5 look-a-likes that looked like the MP5 but did not share the HK's rotating bolt design.
I own a Bobcat BW89PDW, their version of HK's SP89PDW. Bobcats are assembled using genuine Heckler and Koch parts but mix them up with US manefactured parts in order for their weapons to me 922R compliant. The possess enough US made parts to allow them to be legally owned in the US. The Bobcats are high quality items and are very reliable. I am totally saticfied with my gun and would recommend Bobcat Weapons to anyone looking for the next best thing to a HK MP5 at 1/4 the cost.
Hope this helped.
Just a big fan of guns.