JAG,
The main difference in a burn-out or enameling oven and a knife HT oven is the controller. On the cheaper ones, there is none to speak of. They have a timer switch marked OFF-1-2-3-4-5-6-7-8-9-HI, which cycles the coils on and off from 0% to 100%of the time. You have to watch the temperature readout and constantly adjust the dial if you need accuracy. They also often are not as heavily insulated. The basic knife HT ovens have a controller that can be set at an exact temp, and will heat up to that and then turn on and off to maintain it. The next step up is a programmable Ramp/Soak controller. This allows you to program many steps in the HT along with the times to be held at those temps. Some even have alarms to tell you when it is time to quench.
Like anything else, the more complex, and the more whistles and bells...the higher the cost. What many folks discover is that by the time they convert a cheaper used oven to be suitable for controlled HT, they could have spent only a little more and bought a new knife HT oven. The other thing that is quickly realized is that the chamber size often isn't big enough.
