600 deg thermostat
Ironically I am in the same boat, and have started down the same path by chance. I have to control several metal solution tanks, at various temps between 140F-195F. I almost always learn something new when finding related "whats-its" on ebay. In this particular case what began to form from head scratching, and ebay searching, wound me down the same path these other guys are suggesting. VERY cool stuff.
Here's where it got interesting, TRUE those items are expensive new, but they can be found new on ebay for pennies on the dollar... i.e., Omega controllers for $12(NOS), NEW PID full featured programable types with SSR outputs and near universal thermocouple compatibily. I am in the midst of a crash course on the subject, but from what I can tell you can save BIG money this way.
Example, a 12" x 3/4" dia 1000W 240V 1 ph cartridge heater; normal price $130 (minimum)... ebay, $18 delivered. Plus;
Type K or J thermocouple w/ teflon leads (at 600F you might need glass/glass leads w/ SS braided sheath)and compression fittings, all stainless, made to order by ebay vendor), $18 delivered...plus;
SSR, $5 + $5 shipping...$10
Digital temp controller w/ free "K" Thermocouple $45 delivered (max)
So for less than $95 (depending on what you are heating, predicated by a good set of plans and homework) you can start to custom fabricate a state of the art temperature controller that is capable of multi mode, fast switch, algorithm based, any combination... etc temperature control. It sounds scary, and YES it was over my head a month ago, but with persistence you would be amazed. You can build a system that would cost potentially thousands ready-made, and best of all YOU design, repair, maintain, and tweak it to YOUR needs, rather than visa-versa. And repair parts are always available, as well as PDF files from the manufacturers specifying lead gauges, input voltages from thermocouples, resistance values for port selection on the controllers t/c inputs. Etc.
I am excited to try these things out, if for no other reason than to once again conquer the myth that most technology is too expensive, or difficult to fathom.
It is obvious to me that after proper component selection, and careful bargain hunting, the rest is only slightly harder than programming a VCR, and building a bike from scratch. I hope you stay with it, as I will be hoping some of the advice you get from the vet's will help me too.