For the stats, all main stats can be maxed via repeatable events that increase them if you want to
Physique: Training in New Texas.
Aim: Pool, also at New Texas.
Intelligence: Playing chess with Semith and talking Books with Syri.
Reflexes: Run on track at New Texas gym
Willpower: Training with Mirrin and taking Curdosnwehn whatever, the anti-Bothrioc oil drink you get from Charles, also training with Ramis
Not sure about specific builds tho, so I can't help you there
Merc is pretty much just ranged or Melee, ranged is generally better imo.
Kineticist is broken with the ice sheet and just freezing up stuff.
Tech Specialist is broken if you go shield techie, you can't die and if the shields break they die as long you have Bimboleum or Salamander.
no one cares about smuggler.