Are you sure it has to be updated to sell the house though? About 10 years ago some friends of mine bought a house that still had the original 100A plug fuse panel. A few years later I replaced it with a new 200A breaker panel and service drop, fun project, came out really well. Previously there were a few grounded receptacles but most of the original circuits were not, and only one bathroom had a GFCI. This was of course taken care of in the upgrade but we could have left the whole system alone.
Unfortunately if you do mess with anything, you have to bring the rest of it up to code. I had planned to replace the rather small and aging 200A breaker panel in my house with a new fullsized panel but I'd have to update so much else now that it's not really worth it, easier to get by with what's there.