“The house we wish to buy was built in 1970 and has wood double hung windows.
They seem to fit pretty well so we think they will perform well when supplemented with storm windows. Would you recommend this or would you suggest replacement windows?”

From an energy standpoint supplementing the windows with storm windows is
usually the best value for your money. However, older (pre-1978) painted
wooden windows are likely to have lead based paint on them and as the sashes are
moved against each other the paint abrades and can may create lead dust. You
can check the paint for lead and if it is present, I would lean toward replacing the
windows and getting rid of this worrisome concern for good.

“We had our windows replaced a few years ago and now I am noticing what appears to be a bit of fog within the double glass itself. Is this a concern?”

Yes, it is. This could indicate that the seal between the inner and outer layers of
glass has been compromised or lost. Over time, that fog will increase to cloudiness, which in turn, will gradually become unsightly. This is an excellent argument for buying quality windows when you upgrade. Many windows come with a warranty against “lost seals” so notify the manufacturer (keep your receipts) as soon as you notice a problem.