Hurrah! I managed to complete Scenario 6! It seems the trick is to sell off some of the “lesser” units once you’ve built the more expensive units that supercede them. That means you still get money coming in, but don’t have to keep forking out for MORE POWAR. I also found you can sort of “overclock” the power sources, and squeeze a bit more juice out of them.
However, Scenario 7 was a bit difficult too. I almost managed it, but just needed another 20 seconds or so to implement the last generator required. Bah.