Larry,
Allow me to add my two cents worth.
I would buy two mandrels, an A and a B. These will accomodate all kits and bushings you may choose to buy in the future. My personal preference is for Berea mandrels, other members will have other preferences.
Each kit will come with tubes and the other components needed for the pen.
Each different kit will require different bushings. Since the final pens are different shapes, they obviously require different bushings to guide you to the final shape.
As you branch out, you will find that different kits require different drill sizes. For example, the El Grande kit requires two different drill sizes as do other kits.
Not to push one supplier, but CSUSA offers a starter kit for some of their pens, which usually includes 2 kits, bushings and drills.
With respect to barrel trimmers, look for one with a 3/4 inch cutter head. This will allow you to use the same cutter head on larger pen kits. Some suppliers offer a barrel trimmer kit with several shafts that allow you to change shafts for different kits. (Not sure, think Penn State Industries offers this, talk to the folks at Woodturningz for a better price)
Above all remember that for most of us this is a relaxing hobby. In general hobbies pay for themselves in enjoyment, not in dollars. A few people make a profit, dollar-wise, I sell some which is nice, but my profit is in the turning and seeing beauty appear on the lathe.
More than two cents worth I guess, at least in length. Enjoy, ask questions, and feel free to contact me privately if I can help in any way.