What you want is an HK94. This is the semiautomatic version of the MP5. It was imported from 1983 to 1989. They are still around. There are a few for sale right now at www.gunsamerica.com.
The main differences from a real MP5 are 1) the barrel is 16" to comply with federal law as opposed to the 8.85" barrel of a real MP5, and 2) the receiver has been mofified so that you can't attach a full auto trigger group to it.
HK 94s are pretty expensive, because only about 15,000 of them were imported prior to the 1989 import ban. An HK94 in close to new condition will cost from $3,500 to $4,000. A cheaper alternative is a clone, such as an SW5 or BW5. These cost about $1,500, and are of FAR lower quality than a real HK94. I personally wouldn't own one. There are companies, like Ohio Rapid Fire, that will work on these to get them closer to an HK94 both functionally and cosmetically, but they're still several steps below the real thing.
BTW, the Navy model you mentioned is called the MP5-N. It is the submachine gun currently issued to Navy SEALS. It differs from the standard MP5 in that it has a threaded muzzle, and a 3-position selector rather the 4-position selector commonly found on modern MP5s.
www.hkpro.com
If you want to learn more about MP5s and HK94s, a good place to start is
www.hkpro.com.
If I told you a bunch of stuff you already knew, I apologize. I'm not entirely sure what your question was. If I didn't answer it, let me know and I'll try again.