I think your land is fine if you do the following:
1. Sturdy fencing, limbs will come down on it so I'd look into rail/pipe.
2. Hire someone to brushhog it - clear out all shrubs and trees less than 10in. In
Diamater. I would also thin out the smaller trees to allow light and maybe you can get some grass to grow.
3. Cut all low branches off trees.
4. Build your shed and allow room for a small paddock in case you need to keep the horse up.
How many acres? Do you have access to trails where you can ride? How is your yard, can you ride around it, lounge, train, exercise? Do you plan on having a buddy for your horse? Some horses are fine being alone, but I wouldn't recommend it they could get depressed, will be lonely. I'd get a horse unless you just want a mule. Hills are fine for horses.