at your size, and intended purpose of the bike, steel is your friend. it wouldnt be surprising if you cracked older lugged steel racing frames, but a modern lower end steel tig welded fixed frame is going to be much, much more durable than those frames of old with questionable history. people have literally toured around the world on surly steel frames, and almost any serious touring bike you'll find will be made of steel for its rugged durability.

do yourself a favor if you're mostly commuting and errand running on the bike - find something that can take a rack and fenders, that makes all the difference when you need to ride every day. i personally own a windsor clockwork, in addition to 100 other bikes, and i think it would be perfect for what you describe. heavy, bombproof tubes, plenty of tire clearance for bigger tires and fenders, and dirt, dirt cheap.