You mentioned you have replaced the seat switch, but there maybe other safety switches , such as the brake switch, that may also cause the problem. try make sure all safety switches are in the correct position and their circuits are not shorted?
For me I would recommend John D x700. It has really comfortable seats and run smoothly. My machine is a seconded-handed, received from my brother and has been used for like 5 years. Just spend some time on changing the blade and air filters, no other issues