You'll need the extra power and durability. Make sure the belt is a comfortable width. Most gym models are around 18 inches wide and 55 inches long, a size that seems to accommodate people of all shapes and sizes. The deck that the belt rides on should be at least one inch thick to lessen the chance of it cracking should you step on it accidentally. This won't add much to the overall price, but will save you having to lubricate your machine every month.
Most treadmills have an emergency shut-off system.
Understand how the system on the machine you're considering works. If you should start to fall, you need to know that the machine will stop. Other fancy programs may or may not be of importance to you. My previous treadmill had a timer and miles walked program. To me, this was important.