Mebee a little clarification is in order, as this is perhaps a rather nutty idea.
For a homebrew campaign where we players are all creating the continents and races, I'm designing a race of Automatons. They are "born" in factories with pre-set protocols and abilities based on their functions (citizen, soldier, administrator, diplomat, etc). Automatons, by and large, exist to further the interests of the whole- they have no true sentience- at least, they are intended to not. Automatons are capable of learning rather broad skill sets through the use of information spindles (which are created and bought just like spell scrolls are), and are capable of eventually exceeding their programming. In effect, they advance to the perfection of the machine, and then push past it into the freedom of character levels.
Constructs enjoy immunity to mind-influencing effects, poison, disease, immunity to crits, subdual damage, ability damage and drain, and immunity to death from massive damage. What I would like to do, is spread this out across several "monster levels" (well- gaining HD instead of class levels basically) until I met the ECL requirements to take on class levels.
I'm sort of looking for this:
Any player could start as an Automaton, but would be required to level repeatedly as a construct (rather than as a class, gaining nifty construct things per the MM) before being allowed to expand into a personal preference of classes. At level 1 my construct character begins with an Immunity to Subdual, 1d10 hp, and base saves for a 1 HD construct. At level 2, I gain 1d10 hp, an immunity to mind-influencing effects, and etc etc for monster advancement. The construct continue gaining monster levels until he is ready to take class levels. The number of monster levels required is dependant upon what the +ECL is for a construct.
Suggestions for creating this progression (HD to construct abilities) and a suggested number of hit dice necessary to match the +ECL would be most helpful.