What should I look for in a light therapy device for sad?

Light source. Get a light that emits frequencies mimicing sunlight as closely as possible. Make sure it's easy to use under a range of circumstances. If too large, heavy or awkward then a tendency to not use it defeats its purpose. Some run on rechargeable batteries and are quite portable and easy to use. Led lights work well even though they are not totally sunlight similar.
10.000 LUX. The key will be a light box that is 10, 000 lux to be effective. Look on line and there are multiple companies that sell this. Also use properly in the morning for a max of 30 minutes.