Look at the pictures on the "Vintage sewing machine shop--come and sit a spell" on the "main" part of this board and you will see over a thousand pictures of peoples old machines refurbished by the board members. And then under tutorials search for the ones on cleaning up old machines. They are mostly posted by "lostin51" so you can do a search that way.
If you are a little bit handy, and don't mind using a lot of "elbow grease" (energy) up working on the old machines they are well worth doing. Make sure that the base you get has the peddle and flywheel, and that the machine has the shuttle and bobbin, or if it is a little newer, the bobbin case and bobbins. (You can get parts usually, but it is easier if they are all there to begin with.) White and Singer machines are the easiest to get parts for. Have fun!