One of the Dell x51's - preferably the 600Mhz one if you just want a PDA.

Otherwise one of the new HTC pdas, like the Imate Jasjam if you want a PDA/Phone.

Headsets usually use a 2.5mm connector, or you can use a bluetooth headset / headphones.

The 6315 would probably work fine for Skype, as long as the cpu isn't too slow. Give it a go.