thecleaner,
If you are planning on converting it to a MP5K go ahead. Otherwise, well, I can't figure out what good they are.
It is too big and heavy to shoot as a pistol with both hands around the pistol grip. There really is no way to shoot it accurately holding one hand on the grip and the other the magazine.
If you add the K grip - be prepared for a stay in the federal pen. Very illegal.
I like HK. But the SP89 has no use in it's native mode. It requires conversion to SBR (Class III tax stamp and all) and addition of the folding stock and K Grip. Actually, I don't think you can do those things either unless you marry it to an auto sear. Since doing so would be manufacturing a new Assault Weapon (sic).
So basically, you have a weapon that can't really be used for anything unless you add the auto sear ($4000), K grip and folding stock ($500) and tax stamp ($200) and another ($500-1000) in misc to make it all work. Total $5500 plus your SP89 minimum. Course, then you got a really cool MP5K submachine gun.
Having said all that, I think they are rapidly escalating in value. There aren't many of them here, and more are converted to MP5Ks every day. Kind of a rarity to find a clean one now.
James