Xilinx FPGA and board selection help

it is my first time i am buying fpga and its board. i wanted to ask few 
question on selection of fpga/board.

which fpga+board would you recommened for the following work:
1) need to generate frequency  dc - 500MHz
2) it would be nice if ADC and DAC are implemented in the FPGA/board(not 
sure)
3) supports DDS
4) least expensive

thanks!!