Microcontroller board with ethernet for computer sensors
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.