what should look for a super inexpensive single-speed of fixed gear commuter...
 

It looks like I'm going to solve my-lack-of-a-bicycle-for-a-few-months-problem by going n+1 (so I'll have a second bike when guests come and visit me in England).

I'm looking at super inexpensive bikes with flip-flop hubs for around 300 USD bought here in Copenhagen, so I can get the fit worked out and ride it around before buying it.

They seem to be spec'd very similar to these:

http://www.retrospecbicycles.com/b-series-fixie.html

with different stickers.

What things should I be looking out for besides fit and finish? I've never had a SS/FG bike before, so do I need to look for a particular type of dropouts, etc...

actually, i think it is this bike

http://www.nolobi.com/shop/green-white/

guardian article:

http://www.theguardian.com/environme...r-bike-no-logo

It seems to me that the color scheme is the most important mechanical factor of modern SS/FG bikes.
Flip flop hub is big bonus if you are just experimenting with fixed gear for the first time.
Then the gear ratio should be considered, but you can always change the cog, it's not expensive to change.
If you buy a bike that is designed for use as a SS/FG you don't have to worry about it having the wrong type of dropouts.

Fair enough.

Just buy it, run it and put stickers on it I guess.