The older Fichtel & Sachs "torpedo" three speed hubs were well regarded and the older ones were generally considered indestructible. If that's what you have (as seems likely), you should probably keep it. But you could sell it on ebay. I don't know if the early ones have a date code, but the later ones certainly did; so you may find the bike's date through that route.
There are good internet sources on these hubs, by the way; but I always have trouble finding them since they are in German. Here's a couple:
http://hubstripping.wordpress.com/
http://www.3gang.de/3-gang/mod53.htm
You can even get replacement parts:
http://www.velo-classic.de/

What size are the wheels? 28" is not specific enough; read this.