How to go about determining how to wire a breaker box in a cargo trailer

This may not be worth the extra expense and work of installing a sub panel. Looking up some prices from the blue hardware store:

6/3 roll 50 ft       $102        12/2 roll 250 ft   $48
subpanel              $60        3 breakers         $20
3 subpanel breakers   $20        boxes and outlets  $25
main panel breaker    $20
12/2 roll 50 ft       $25
boxes and outlets     $25
-------------------------        ----------------------
total                $255        total              $93

I think 3 circuits is appropriate. One for the tool you're actively running, one for dust collection, and a spare if you need to be running two tools at once.

Edit: If you plan on upgrading to tools that will draw more than 20 amps, it may be worth the extra expense of putting in a subpanel. That way you can use shorter runs of 10/2 or 10/3 from the subpanel to the tools, instead of running all the way back to the main panel.

Match the letters

If the breaker has the same "Type" letter markings on it as are listed on the panelboard label, you're fine. Number suffixes on the type marking (such as QAF2 vs QAF and MP-AT2 vs MP-AT) are used to denote a revised type of breaker that is still compatible with the original and can be used in place of it, so they don't matter in your case.

10kAIC is fine

The Siemens/Murray line of breakers comes in three interrupting rating options -- 10kAIC, 22kAIC, and 65kAIC. The larger options are more costly due to extra internal equipment needed to break bigger arcs, and are only necessary in commercial work where larger services and feeders capable of higher fault currents are found.

In addition, in a main breaker panelboard such as yours, a breaker with a lower rating can be used because it receives a higher series rating by way of being "backed up" by the panelboard's heftier main breaker.