If I see an interesting old farmhouse for example w/no address: 1. would I go the local tax office and use the closest address I could find near that house to find who the owner is?
2. If that is the right starting place, does that mean the county would want all the back taxes from me plus a sale price as well if the house has been seized?

Yes bring the closest address and maybe the 2 closest cross streets. The county assessor will have owner's information on file and it is public record. I have found them to be very helpful. If there is a tax lien, yes the county would require that to be paid. The back taxes would be a lien on the title and you wouldn't get clear title to the land until they were paid.