They are nice, but if its not for collecting.....I don't see the point, a Like new SP89 is going to run you $2500-$2800, with new ones in the $3000-$3500 range. Plus, they don't look half as cool as an MP5K...some Terry Dyer and Vollmer conversion guns out there (based off SP89)- but are super expensive.
If you want an assault type pistol without costing a fortune (I'm not comparing them too an SP89)...but an Uzi 9mm pistol is lots of fun and reliable, the Steyr SPP is nice also...both shouldn't cost anymore than a $1000, SPP might run slightly more.
But if $ isn't a concern, then more power to you. good luck.