TM's are generally speaking very reliable indeed. The older they get the more likely you are to have problems, but then that is true of any piece of machinery. The lifespan of a TM is often down to how well it has been looked after throughout its lifetime, but you can generally tell that from its appearance and how well it runs, how quickly the heat exchangers pick up heat (and loose it when the trigger is pressed which determines the temperature it will maintain in use), how smooth the pump is and how generally the machine sounds. The only way of determining that is to try before you buy.
Simon