Re: City won't buy enough water/power to meet demand from other regions

No, there's no limit. You should be able to buy all the power (or water, sewage, etc) available in region view. The problem of course is related to slow server response in trading resources between cities. Unfortunately at the moment there's nothing you can do but wait. The city will eventually buy all the power it need, but it might take a little long. Hopefully EA will speed up server response soon.