Since there seems to be some confusion over what you have exactly. I would call a service company out to take a look at what you have. It sounds as if you have a fixed speed that is adjusted by moving motor speed taps to where you want the speed to be. If so, purchasing a thermostat to dehumidify will be a complete waste of money, it won't do anything except cost more money. If you have a true variable speed, which runs at a range of speeds and has a control module piggy backed on the actual motor, then have your service company check into what you need to make it work properly. I will say at 14 years old, I would also make sure it has the capability to run your new system. You could develop a coil leak or failed motor which fixing would be fairly expensive and a waste of money.