If you've got the money, go ahead and tear them down and start over. Everyone likes the new barn smell! But if funds are limited, I would put them into the horse barn, and fix up the tractor shed so it is stable and weather proof. You can build a new tractor shed when you have the money.
I have a barn that was built in the early 1800's from chestnut wood. Held together with wooden pegs. I am working on getting it suitable for horses. Have replaced the siding on 2 sides, with one side left to go (one side is good, and did not need replacing). I find old single pane windows on Craigslist and add them to the barn as I redo each section. The nice thing about working on a barn is it is a barn - if things are not plumb, or you make a mistake, the horses won't notice! I use it to learn new skills and gain building experience. One of my first things was to paint it red with white trim. Makes me want to keep it up and in good shape, and I figure the paint strengthens the building! If it has not fallen down in 150 years, it will most likely last for my purposes!