I just bought a 70 year old home in somewhat of a lower-end
neighborhood that I am fixing up a little, renting, and then maybe
selling it in 5 to 10 years. It has hardwood floors that have been
covered with carpet for many, many years. If I decide to sell the
house several years from now, I will completely refinish the wood
floors. But until then, I would like to find the cheapest method to
clean them up and protect them through the rental years. Can anyone
out there suggest some possible solutions?

My first thought was to lightly sand the floor, apply a darker stain,
then a good thick coat of poly. But I'm kind of confused as to
whether or not that will work if my current floor was shellac or
varnish (probably the case based on the age and condition).