What to look for when buying a single gear bike? I'm looking to buy a single gear bicycle. My experience with bikes is very limited, I haven't owned a bike since I was a kid. I have been mountain biking in the Lake District a few times in the last 3 years or so and enjoyed it a lot. At that time, I just hired a mountain bike.

I would like to buy a bike to to commute to and from work (8 miles/13 KM each way - no steep climbs). I'm tempted by the single gear bicycles because they require low maintenance and fitness wise they can be more of a workout.

My criteria so far is:


Has to have a free wheel, 
Chain tensioning mechanism,
Brakes on both
wheels


Is there anything else a newbie like me should consider before buying a single gear bike? I'm 6'2" and weigh 10 stone.

Appreciate the help.
Some other items you might consider:


Most importantly, the fit of the bike. If you are 6'2, you will be on the high end of normal sizing, and not all models may be available in a suitable size. Staff at any decent bike shop should be able to measure you, and suggest a range of suitable models however. Take them for test rides and make sure you don't experience knee pain or back pain. 
Weight of the bike. If you will ever have to carry the bike, this can be quite important. For example, if you live in a flat above ground level, and don't want to leave your nice new bike outside at night where thieves can get it. Single speed bikes tend to be lighter anyhow, so you should have no trouble finding a cheap model that's still under twenty pounds.
Low maintenance design. If you want a low maintenance bike, single speeds are a great start. I'd also look for a coaster brake (back-pedal brake), and a nice chain case, possibly even a full chain case if you can bear the weight. An aluminium frame may also be a plus, as ChroMo (Steel) will rust relatively quickly without careful maintenance in wet climates, especially if there will be salt on the roads in winter. 
Fenders if you ever plan to ride in rain, mud, or snow.
Rack Mounts if you ever plan to carry something heavier than a backpack. Also useful for riding in the heat.