ive heard of 3 thats have gone over 220k, do a search for user name Sprint16v he had a williams 3 from new to mega miles and was still fine will the belt snapped, Paul Mitchell bought it and stripped it and the bottom end was fine, remeber they dont rev very much and do tend to be looked after. There ois also a web site where a mechanic re-built a 220k one and its was basicaly fine, very little wear considering.
the shape of the williams engine is more stroker diesel than screamer racer and they therefore wear accordingly, The wear is going to be more significant in the rings/bores then the equivelent 16v for obvious reasons.
And to boot i put my money where my mouth is and bought a williams a few weeks back on 109k, history tells the life of the car and its went to 90k very quick (3-4 years) with renault servicing and belts etc, and from then on its lived a very sedate life.
remeber all parts are basicaly cheap because its from the clio 16v/R19 16v parts bins, only thing to really worry about is the bottom end but as stated above it shouldnt put you off.
How much is he aksing? (if you dont mind me asking)
Very good time of year to be buying mine was a steal at £2800 for a 1995 N williams 3. Investment!!