Grab your Dungeons Master Guide, and a Dragon Star "Starfarer`s Handbook".
Look at the Advancement Rules, than look at the Robots. Remove the Rule that Robots have no Constitution Score, give them a Standard Score of 12 for Medium size Robots. Now, increase size to get the different Mechs with different Hitpoints.
You might decide that every Mech has a Base Attack Bonus like a Wizard. (Half Hit Dice), as a targeting system or something like that...
Mustrum Ridcully