FPGA board

1 - It's difficult to know if the FPGA you've selected is "big enough" (has enough logic elements). That depends entirely on what you are going to do with it. Typically what I do is try some test projects first with what I think is a fair representation of the amount of logic I'll be using and make my decision based on that.

2 - The FPGA should be handle all of the interfaces you've listed. You will of course need external components to support ethernet and RS-232.

Are you sure you don't want to use a dev kit? Like this one:

http://www.altera.com/products/devkits/altera/kit-cyc3-starter.html

You could just create your own mezzanine board and connect it through the HSMC (available from Samtec).

Jake