Electronic – Microcontroller with both ethernet and radio onboard

Microchip's PIC18s with built-in ethernet are excellent for this, just add a magjack (or other connector with built in magnetics) and download their TCP/IP stack. You'll be pinging things in no time. For more grunt, the PIC24 and PIC32 also have TCP/IP stacks designed to run with one of the SPI Ethernet MAC/PHY parts they offer (ENC624J600 or ENC628J60).

ST also recently enhanced their ARM Cortex based STM32 line to include on-chip ethernet, but you'll need an external PHY and magnetics. It's an option if you're already looking at the PIC32.

For a one time board, I think an Arduino with a ethernet shield is going to be your easiest path for you. If you are wanting to produce and market you should look into a more specialized/customized solution for the aspect of cost.

Placing the board inside is good because you will be keeping your SPI lines short and have power close by, but putting your board outside will give you less of a concern of shorting to other electronics in the server.