not sure on a mondeo abs ecu; but i bought a audi A4 avant with the abs light on as it was cheap; Audi wanted £1052+VAT + Fitting for the new abs ecu.

i found a new ABS ecu for £600 inc VAT and fitted it myself.

ideally if you can take a mechanic with you with a scanner, you can see what the fault code is and then make a decision from that.

Also maybe be worth checking if the ABS pump & ECU are one combined unit, if they are and it does need replacing you're talking more money cos you've got to bleed the system in after fitting a new pump