The books advise to not start the APU because if it is unsuccessful, you will have severley limited your time available on batteries only.
Even though the engines may have been damaged and flamed out, if the core is still rotating, the hyd pumps may have been producing some pressure. If we paratice a single engine landing in our 320 sim, the hyd pressure for the windmilling engine drops to the ECAM trigger on approach. Wheras, my colleagues who have done it for real say that the windmilling engine will produce sufficient pressure down to about 70 knots.