While you CAN buy your own rackmount chassis (like a SuperMicro 2U) and put in standard desktop components in, for what you describe, I would highly recommend just buying a pre-built server. For one, since you said you require 2x Gigabit NICs, most pre-built servers come with this onboard, whereas most desktop motherboards do not.
I would say maybe take a look at the Dell PowerEdge R210... starts for as little as $899.
Dell PowerEdge R210 II Ultra-compact Rack Server | Dell Canada