Would you advice buying a D800 for shoting 50% of pics in not ideal conditions?

Not knowing your exact condition, I have an 800E that I shoot everything with, Sold my D4 because of non use over the D800E.

You have the high ISO to compensate for "camera shake" just shoot faster speeds. Yes you will get more grain, but with your conditions you have no choice if you want high MP.

The camera will never stop surprising you. I shot a Track meet the other day that I would normally not think of taking such a "slow" FPS camera, but not a problem. Instead of a 20 burst shot I did  a five burst shot and still make the picture I wanted/