You could fly and turn with only 3 or 4 rear facing thrusters, though you could not turn in place or move sideways. Ships in SB require differential thrust to turn, so a ship with 1 thruster could at best fly in a straight line from when it launches, unless you could aim the ship before launch it would just move out in a random direction. With 2 thrusters it would be limited to a 2D plane, but space is 3D. With three thrusters it could turn properly, but turning could be confusing as your three options are at 120 degree angles. With four you finally have a button for each left, right, up, down; which makes aiming with WASD much easier.
I would go with the missile launcher as it can unload all of the ammo in a few seconds but do a significant amount of damage in those few seconds. An exposed pilot seat above the launcher, then a battery, fuel tank, and four maneuvering thrusters behind.
A dedicated ramming ship is possible, but even the smallest, most minimalistic ship is going to be vastly more expensive than a torpedo I think. Though it would have vastly better range. Of course the enemy could easily shoot down those small cheap ships, so you might need several. You would need a lot of explosive power to justify the cost, or noted above, a boarding party.