Your requirement for the pure sinewave at high power is what drives the suggestion that you use a generator. It is quite simply the best way to go for that requirement. DC from the battery drives a motor which drives the AC generator, and minimal electronics.
Because you are going from low DC voltage to a high AC voltage means that you will be moving high currents. Heat goes as the square of the current. If you use in excess of 50V on the input you will have safety concerns on the input side as well as the output side. I regard this project as highly impractical. If you do find someone to do this, I'll be fascinated to hear how they made it work.
I believe your best bet is to buy the kind of syncronous inverters that the solar and wind power people buy for the purpose of selling power back to the electric utility.