IMO
Turning pens is like anything else, you can go as cheaply or as expensive as you want.
Mandrel, I prefer a standard no frills mandrel. if I need to adjust for length, I will pact the extra space with an tube or bushings to achieve proper working length.
Drill Bits, I started with a PSI bit and it worked fine for many pens. Once it dulled to the point of replacing I opted for a Colt brand. I have heard that the Fisch is supposed to be very good and less expensive.
Barrel trimmer, For a while it seemed it didn't matter which one you bought, they were all the same "junk" having to apply way too much pressure to get them to cut and they dulled quickly. I have upgraded to the (more expensive) Whiteside barrel trimmer. It's like night and day.
Links are what
I have, you decide whats best for you and your situation.
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A lot of this depends on how and what you turn, I ONLY turn wood, no acrylics or man made materials. There are special bits and methods for those products.
One other thing, if you can, when turning 20 pens I would build them in large batches, at least ten at a time.
Do all the cutting, all the drilling, all the gluing etc, in an assembly line method, it will save you time overall.