I did have to buy a pen turning mandrel and i would highly recommend the Rotur Pen Mandrel Kit, comes with all you need including a 7mm drill bit, barrel trimmer and adjustable mandrel for doing all lengths of pens.Anyway that's just my opinion on pen mandrels...
I didn't exactly know what sort of pens i wanted to make but nearly all the wood turning shops sold the classic 2 piece twist pen kits so i went with them and bought about 10 kits at €2.50 each, they were gold coloured pieces and looked nice enough in the packets anyway :D 
