Old thread, but ...
@ Cyberface: How To: Adding an OEM Tachometer Cluster on a 2005-up
You will want it to be from an 05-07 Focus, the 00-04 won't work.
ABS and TCS are the main issues (and mileage). If your car doesn't have those, you could have an indicator light on on the new cluster. If it does and the new cluster doesn't support them, you would have no failure indication.