Look at second hand bikes. No disc brakes, no suspension, old but working gear and full mudguards (mismatched if possible :-)). I had a Marin Bolinas Ridge. Steel, grey, so looked very plain. Mismatched wheels, I did put v-brakes on it as cantis were pretty bad in wet, no a-head stem, probably best to not have anything with qr's. The bike worked spotless, 1x7 acera/alivio, new cassette+chain was less then a chain for my road bike, bike looked shit but was only 3min slower over 6 miles than my road bike. Put some puncture proof tyres and you're ready to go.
Those Edinburgh courier bikes are pretty good, stock wheels are a bit weak, however I did have a play with one, and can't really fault it, maybe it looks half decent for a commuter so it would probably got nicked quicker then my Marin.
I think everything depents where you will be able to keep the bike. If it's indoors, you can have something working good but also looking ok, if it will be locked outside then it really needs to be something looking shit. If it will be pig ugly, the only risk will be some neds will want to kick the crap out of it for fun.
check this out, for 60 notes you can't really go wrong. 7 speed and has v breaks so will stop reasonably well and be cheap to run, it might even look too good.