Here is my "expert" answer. I am qualified to give advice because I have made three pens so far, but I have made or re-made each of them at least three times. That qualifies as 9 pens at least.
I recommend the following:
For #5 size - 9x.75 for section to body and 11x.75 for cap to body
For #6 size - 10x1 for section/body and 12x1 for cap/body
The choice to use .75 threads vs. 1.0 threads is optional in all these cases.
As soon as you make one you will wish for a triple start T&D, but they are very expensive and they can wait a while.
The feed housing you buy will dictate the tap size for it.
Bottom line - you only need two sets of taps and dies plus one extra tap for the section if you just pick one size nib.. Total cost is under $100.
I am using a collet chuck and wishing for some kind of expanding mandrell, but I am not there yet.
Hope this helps.