The one piece of advice I can offer, and it is the most essential as far as I am concerned, is to buy a thermometer with a detachable, moreover replaceable, probe.
Some models have the cord and probe permanently attached. Some configure the hardware to allow for replacement of the cord and probe in the (inevitable) event of a failure. Think of it in terms of a DVD/TV combo; one of them will die, don't force the other to carry around a conjoined limb.
I recommend you look for a well rated device (shop around) that has a detachable probe. When the cord gets a short or the probe goes bad, you can swap it out for less than the cost of the whole unit. Aside from that, brand and cost and features are up to you.