Dan, are you buying the mandrel for yourself to turn pens or for someone else that has a lathe and you think they want to get into turning pens?
Let's assume it is a mandrel for yourself and your lathe. Which morse taper your lathe has (1 or 2) determines which taper you get on your mandrel of course. The price of the adjustable mandrels is very comparable and for different pens or projects requiring a mandrel it is quite useful to be able to adjust the length of your mandrel so I would suggest an adjustable mandrel. Common diameter 7mm.
Again, if it is for you, pick a pen style. Slimlines and or Saturns are quite easy to turn. Since you mentioned PSI, they generally have a starter package that is a little more economical as it includes 3 or 4 pen kits, the correct drill bit and the correct bushings. With this starter kit and the appropriate number of pen blanks, quite easy to get into the pen turning vortex.
God and family, the rest is icing on the cake.
I'm so far behind, I think I'm in first place.
Premier Bovine Scatologist
Host of the 2015 FAMILY WOODWORKING GATHERING