Make sure you get one with a photo on it, as you need a photo if you want to put a season on the Key.
Oh too late
No, you wont need a photocard when travelling on a Southern service, however if travelling on another operator they may request to a photocard or another form of personal identification may also be requested when travelling on other Train Operators as per the National Rail Conditions of Carriage. Relevant photo identification will also be required to support some discounted tickets e.g.16-25 Railcard.
Do I just buy a ticket (East Croydon - London monthly) online as I usually would and then touch in to load?