How to choose FPGA for converter control?

You can design your own comparator & PWMs using Verilog/VHDL. It is so simple, if you know these languages. However you may use Megawizard from Altera Quartus to generate comparator. There are on-line codes also available.

For the FPGA part, I think even the simplest parts will suffice your application. Your timing requirements are not so tough. A CPLD I think is enough.

You need to design your comparators/PWMs first. Then synthesize them using Quartus on some CPLDs. Finally you will know which part fits your requirements.