“is IPC a unit of currency which carries over into the next turn? i.e. I have 10 IPC and I spend 8. (assuming I neither gain nor lose territory holding IPC etc) would that mean I have begin the next turn with 12 IPC?”
Yes, you can save the money. You can’t give it to another player but you can save it for later. Having some of the old paper IPCs from earlier games or some monopoly money can make this easier because you can use the IPC tracker as usual but you get a paper coupon for anything you don’t spend. Sort of like cash in had (paper notes) versus in the bank (IPC chart)
“Secondly: Industrial complexes, can they be destroyed? Or only damaged?”
Only damaged, never destroyed.