It has come to the point where I need a torque wrench for installing things on my bicycle. What should I look for when purchasing one?
You want to make sure that it will serve the range of torques that you will realistically be using. Most bicycle tasks will require quite a low torque (e.g. 8 Nm) but occasionally (crank bolts, pinch bolts) you'll need much higher torques (e.g. 50NM). I have 2 torque wrenches, a 'sensitive' one which ranges from about 5NM to 20NM (in fine graduations) and a medium duty one that ranges from about 20NM to 100NM (in broader and less accurate graduations).
It's handy to have measurements in NM and lbFt, but I'm sure every torque wrench on the market will display both values.
Most torque wrenches accept square drive sockets but the really swish ones also have adapters that let you use open wrench heads. This is great for places, such as the pedal axle, where you couldn't use a socket (Beta do such ones).
If you already own a lot of square drive sockets, then you may want to get a torque wrench that accepts the same size.
Tip: ALWAYS unwind your wrench after using it. Storing it 'torqued up' will affect it's calibration.
Some jobs require working with left-hand threads, for example, fitting cups to a BSA-threaded bottom bracket or fitting pedals. This is more likely a requirement for your medium duty, higher torque wrench.
Some torque wrenches advertise 'reversible' ratchet heads but this is not the same thing. Unless it specifically says so, the wrench is unlikely to work on left-hand threads.