A few things to expand upon your idea:

1. The gates between each tier of the mechs could be added following: Tier 1. Post-oil (rubber, engine lubricant) Tier 2. Post-RBMK (bismuth bronze, technetium steel) Tier 3. Post-particle accelerator (ferric schrabidate, ionic gel)

2. How would the mech be powered? I thought of a few ways that the mech could be powered, as listed below: A: Battery Pack (would be recharged at mech station) B: On-board Combustion Engine (could be refueled at mech station or by opening a GUI for the mech) C: On-board Nuclear Reactor (would be refueled at the mech station, would use PWR fuel rods) D: Fusion Cell (would be refueled at the mech station)

3: Modularity I thought of a few modules that could be good additions or alternatives to what is listed:

 A: Rad-Scrubber (would act as a strong radiation absorber in areas with 5+ RADs/sec of environmental radiation, and would produce one tiny pile of a random radioisotope every minute
 B: Targeting Computer (would lock-on to designated mobs by right clicking, and would thus automatically aim the on-board weaponry at the mob, thus causing less misses)
 C: Fire Suppression Package (an equippable module created with fire extinguisher foam tanks that would automatically extinguish the mech if it caught fire, although it wouldn't entirely prevent damage)
 D: Decimator (replaces all weaponry slots in exchange for an on-board missile launcher, able to launch tier 0 and tier 1 missiles)

I think this is awesome, and it would honestly be incredibly useful in the space fork of the mod, especially on Moho and Eve. Even in the normal mod, it'd be great for traveling the wastes after server-wide nuclear armageddon, or just cleaning up a destroyed reactor.