I think the Cannondale ST400 would be a good choice. It's a 'sport touring' frame so it will be a bit more stable (and might also feel less energetic) than a road or criterium frame, but it's got good clearance for fatter tires/fenders and mounting points for a rack.

A quick search of the internet reveals the Nishiki Continental is a full touring bike. So, like the ST400 above, it will be stable and have lots of mounts for racks/fenders/gear. At $80 it actually sounds like a bargain, and if you fix it up and decide to sell at some point I bet you could get $300 or better for it if it's in decent shape.

Now, if I were riding a hilly metric century, I might want something a bit lighter and with slightly tighter geometry than either of these, but I agree they would be better than a fixed gear and MTB.

One other note: the weak point on many vintage bikes (and even new entry- to mid-level bikes today) is the wheelset. If I were you, I'd factor a new (or decent used) set of wheels into your budget and some decent tires. Wheels and tires greatly influence the quality of the ride.