An older road bike makes an excellent choice for a commuter but when it comes to road bikes there are sub categories... avoid racing and tt bikes as they do not allow for fenders and will not accommodate decent sized tyres.

Sport and touring frames are best as they do allow for fenders, have a more relaxed (and comfortable) geometry, and will run wider tyres with those fenders which wil really improve the ride.

Look for alloy wheels and a square tapered bb and cranks as the lighter wheels will increase speed and improve braking and a square tapered bb and crank is much more serviceable / upgradeable than a cottered or single piece bb and crank.