ABS light: what's the worst it could be?

[Nasco12]

I am currently looking for a decent Merc under £1000. Today I saw a reasonably nice pre-facelift W202 C200 but when I took it for a test drive the ABS light came on. I walked away. The question is should I have? What's the worst it could be?

 

[Howard]

Absolute worst is that the ABS ECU is dead.

Unlikely though.

 

[BlackC55]

ABS pump.

Its probably a wheel speed sensor, Very common.