I personally think the HNT-151 Hornet would be a great starter mech in MW:O.
It is the cheapest mech in battletech.
I has an LRM-5, a Medium Laser, a Small Laser, 150 meters jump capacity, and has the maximum armor a 20 tonner can carry. It is fairly slow however.
For a "starter mech" the Hornet would new players to experiment with everything from fire support, scouting, rooftop hopping, and more. It is also armored well enough to take a hit or three.
Any TRO will tell you about the mechs handicaps. But I think players would enjoy it in game. It would give new players a taste of what piloting the much larger mechs would be like. A Locust or Wasp is generally regarded as superior to the Hornet, but for in-game fun the Hornet packs a lot of goodies into a cheap package.
I'm a closet Hornet fan. Both it and the Urbanmech were designed for urban warfare roles, and perhaps coincidentally, they're both super dorky, in that fun "laugh at it, then pat it on the head and throw a stick for it" sort of way.