I'd be inclined to go for Merchants, since they're the smallest class with a collar (two, in fact) and a grav deck (IIRC). Even if you only plan on using them as a pony express, extra transportation capacity is always a good thing, nobody in battletech has enough of it. I want the grav deck for the long-term health of my crews. Some might consider that a waste because you can generate .1G with station-keeping drives, but I think it's more efficient not to have to be constantly burning reaction mass to keep my crews healthy, and the full G of a grav deck has got to be better for you than just a tenth. Then there's also the fact that the Merchant is the second most common jumper in human space, which might help the Watch out if they ever need to do sneaky things. Everyone knows the Bears (or at least some Clan) are in town when a Hunter pops in, but everybody who's anybody run Merchants.