Well, that depends.
For example, you have a simple wheeled trailer stacked with RL-10s. An engine is required only if the trailer is meant to operate independently, so, no. Now, if that trailer carries an AC/10, it needs ammo. Because it's a vehicle and ballistic weapons don't generate heat for vehicles, you don't need a heatsink or two.
Let's say it's a towed hardpoint for an infantry squad; lots of armor, a couple MGs in a turret, Infantry Bays, whatever. What powers it? You're driving away the generator and TGT computer on your tank. It needs an engine with a rating of at least 10 to power it.
To the best of my knowledge, the mass of the trailer is irreverent. Knock yourself out. *Remember you don't necessarily need to pull it in a scenario, you could just use it as a temporary fortification.
