Depends on budget and the size of the expected Squad.
If sky's the limit, then put the smallest XXL Engine in there, 6-12 tons for Cargo space, 1 Weapon per Facing, and the rest goes to Armor, either FF or Reflective.
But that's more for Fan Design.
Honestly, the Maxim and Heavy Hover APCs can work for Inner Sphere well enough. Bandits are fantastic with both internal and external mounts, and there's both Inner Sphere and Clan options. Only ComStar is left out of this.
Of course, if you really want to have fun, go with one of the Karnov VTOLs. There's either the Stealth Variant that makes it more survivable, or the 10t cargo space one that can drop 2 Squads or Points on a target, without landing (if the BA can Jump, of course).