The Walther PP line is noted for being dependable - and they last a long time.
The original design was for .32 ACP (7.65mm), and later expanded to .380 ACP (aka "9mm kurz", "9mm short"). I have a PP in .32. Note that the same frame was also available in .22 LR!
$300 sounds like a good price for a PPK/S in good operating and cosmetic condition, regardless of how many rounds have been through it.
If you opt to use one of the non-ball-ammo defense rounds, make sure to run several boxes through the gun. All autos have the possibility of feeding problems; make sure yours is happy with that particular round.
More good info here:
http://en.wikipedia.org/wiki/Walther_PP_Kurz
http://www.thenationofriflemen.com/nor/index.php/ggps/walther_pp_k_s_380_acp/
