If you purchase an older tractor that has not been restored, the probability is high that you will have to do a significant amount of restoration to turn it into a reliable machine. This may be an attractive alternative in that you will pay a fraction of the cost that a restored machine with warranty would bring. You are then in the position of being a mechanic. On the drawback side, you will need tools that are large enough to work on the machine (most folks don't keep a 15/16 socket or open-end in their tool box) and space to tear it down.
The pluses of working on older tractors is that they are much simpler than new tractors. The electric's frequently have no modern ignition just a simple magneto (the "circuit" is a wire running to a cut-off switch), the carburetion is gravity feed, the hydraulics are "one-way". Understanding the components is extremely simple when ompared to the modern counterparts. Still a basic knowledge of internal combustion is valuable though easily attained.