You may or may not need chemo or radiation. Depends on the extent of disease, of course. If your care team thinks you will most likely need radiation, it may be beneficial to avoid radiating a newly placed flap, however they tend to tolerate radiation without problem.
That being said, the reason to place an expander or implant at time of mastectomy in a situation like this, is to preserve the shape of the breast skin envelope and its textural quality rather than cutting off all the skin to create a smooth chest closure or just letting preserved skin scar down. This provides the opportunity to avoid having a large patch of skin placed at the time of your DIEP so that the end result looks better and more like an un-operated natural breast.
Hope this helps,