Still interested in doing the pirate thing, and heres my thoughts so far:
First off, the most likely method i can think of for the pirates to get a military class ship (anything less than that is pointless, since it wont be able to stand up to military forces if it comes to blows on occasion) would be for the group to start off as a crew going AWOL, in a mutiny or something similar on a ship.
So far, the most likely candidate for a ship seems to be:
https://stararmy.com/starships/qns/isfb/isfb.html
Due to the low crew number (30 prime crew allows for a more likely candidate for most of the bridge/medical/tech crew to be in on the coup, with only a few having to be spaced during the process).
If this sounds reasonable so far, what is the next step?