Hmm, I like the idea for the mission, but you have one major plot hurdle, the Sabre class vessel has no cloak, so how do you cross the neutral zone without breaking treaty?
My suggestion is that you, perhaps allowing for a lot of fun, get the players to hide and disguise themselves as an innocuous trade species, like the Yiridians or another made up species, who can cross the neutral zone, on a somewhat ramshackle 'borrowed' ship That way they could get a taste for command both in the sense of a ship (without it being a full Starfleet state of the art vessel) and give them some experience at espionage and good 'role-playing' trying to blag themselves out of some tricky situations, if their disguise isn't quite good enough! Depending on what era you are setting it, the Vulcans might be able to contact Ambassador Spock to help the players out, while on Vulcan, or if earlier, maybe some of the unificationists have contacts with the Vulcans, and can serve a similar purpose.
With such a mission they might get some leeway for more 'creative' ideas on how to do things!