If you are considering a treadmill because you are currently overweight, I'd urge you to think hard about your decision.
An exercise like running (outdoors or on a treadmill) puts a lot of impact on your joints. If you are currently carrying some excess weight too, this damage will be multiplied.
There was a man who used to attend my gym, who was overweight looking to drop a couple of stone. He would jump on the treadmill 5 days a week, and despite the above suggestions, he was emotionally attached to running as he believed it was the best method for burning fat.
The result? Within 3 years he permanantly damaged both his knees and required surgery because of the running.
I strongly urge against impact running if you are overweight. Firstly because it can very possibly destroy your knees, and secondly because it is far from the most effective method of fat loss anyway.
Buy a good set of free weights from Argos for £30 and save yourself £670 and the health of your legs. Resistance training will be a lot more beneficial for you.
Im not trying to sound nasty, Im just saying based on the fact that overwieght individuals are not recommended to perform excessive impact exercise.