No, stamp duty on a new house depends on square footage not price.
On the PC sums, one thing to note is let's say with a fireplace:
If you want your own fireplace then the company you get to install the fireplace will have to wait until after you've gotten the key from the builder and as you can imagine its quite difficult to install a fireplace when the sitting room has been finished - floor put down, etc.
What I've decided to do is go with the company who are supposed to put in the standard fireplace and choose an upgrade from their own list so they'll still put it in at the more opportune moment.
For a kitchen, its best to have the whole floor tiled first before the units go in as otherwise groundlevel appliances like dishwashers and washing machines will be virtually impossible to move if they're on concrete and the rest of the kitchen is half an inch above.