Although your doctor should be able to help you to a large extent, there are some general guidelines you can go by, when buying a pair of shoes.
The size should be tried on best in the evening, for that is when the foot is most expanded.
If you are going to mostly wear your shoes with socks, then it is a good idea to try a new pair with socks.
The shoes must provide enough support in the form of adequate cushioning in the soles of the shoes. Which essentially means, that the base should be well padded. This prevents the impact from further weakening your joints/back.
The arch of the shoes should be padded, also to support the area.
The forefront (first 1/3rd) of the shoe should be flexible enough, and roomy to prevent cramming of toes.
The shoes should be airy to provide ventilation. These days, there are a number of lighter materials, that are being used provide ventilation as well as make the shoes lighter, thus less cumbersome.