I used a streamlined version of the 1000 ton "Fortune class" merchant (3rd party design from one of the supplemental booklet publishers of the CT era, but I can't remember who; or put my hands on it right now.) Rational for the design was for use as diplomatic or trade missions that might need to carry a fair sized party and some cargo of some sort (trade goods, supplies, etc) beyond the normal jump routes. I kept the drives as per the std version so it would have J3 (can't remember the other 2). I the reduced the staterooms and low berths, added a second bridge and computer in their place to be used as a CIC (combat information center), replaced the small craft with fighters disguised as "playboy" in system racing craft.
My PCs were to use characters who 1. had valuable skills to teach the trainees, 2. were getting ready to retire, and 3. had performed valuable service to the Imperium (I.E. had been awarded the SEH). After workable characters had been generated, they were invited to a special auction where they were contacted by someone who offered them the opportunity to become part of the school staff. The plan was for them to work with the trainees on skills for ship operation, all kinds of combat, and any "black" skills they might have. Area of operations was the 3I parts of the Spinward Marches. After the class of trainees were graduated, the staff inherited the ship free and clear - subject to removal of any special equipment, etc.