[Enji]

Personally I'd prefer if appearance has little to do with performance of various tasks. I know it's not a realistic view on the whole idea, but getting too realistic with an idea like this will start creeping into the realms of uncanny valley/squick. Some generic stats with baselines determined by your cowgirl's "breed" and how difficult it is to raise a particular stat would be fun enough for me.

Errrr let me try to reexplain, I'm of the camp that all types of cow girls can max out any stat, just a matter of how difficult or long the process will take depending on their body type/breed/whatever.

With drugs, there should be a diminishing results effect. Like the more you feed her a breast growth drug, the less effective it becomes. That way most things shouldn't hit the realms of insanity like barn sized boobs or something crazy like that.

There should also be a quality system for the milk as well. Depending on your particular cow's stats, determines the quality of the milk she produces. The better the milk, the higher a price it fetches. I think this would be able to overcome the hurdle of producing enough money for later game stuff without having to have a gigantic herd of cowgirls.