Sorry it has taken me so long to reply. I've been working overtime and just have not had the time to even turn the home PC on until tonight.
Yes you can use aluminum. Or plain old sheet metal, or go high end with stainless. Just stay away from using any galvanized metal or galvanized screws, bolts, etc.... Galvanized metal will emit chemicals when heated and you can get "zinc poisoning" also known as "metal fume fever" (google it). The stainless looks great, but plain old unfinished sheet metal will work. As soon as you start using the smoker, you will get a coating of smoke on the interior that will protect the metal.
Rock Wool is also called mineral wool. It's made by heating rocks until they melt and it is then spun like fiberglass into a mat. Mineral wool has a very high temperature resistance usually in the neighborhood of 2,000* or more. Grainger, McMaster Carr, Lowes, Home Depot, Menards, etc... all usually carry it. A common brand name is Roxul. It comes in batts like your home fiberglass insulation and in sheets of varying thickness. If you find a good deal on sheets that are too thick, you can usually split the sheets easily to make them thinner (may vary by brand and design though).
I would not go to the trouble of making a separate box, but I would put some sort of grease drip guard over the heating element (can be a simple metal plate held a inch or move above the element by metal stand offs or brackets). If you are cold smoking you will probably just not turn on the heating element (although cold smoking covers a wide range and can extend into the lower 100* range for cured meats).
As to the gear box & motor, you are talking about rotating a lot of weight in meat. I seriously doubt you will find an affordable stepper motor with the torque you need. I would think about salvaging a DC reduction gear unit from another commercial product (e-Bay is your friend for this sort of stuff). You have other options though. A gear reduction drive with a small sprocket that is connected to a much larger sprocket via a chain drive would be the first thing that comes to my mind. Size the sprockets to get the desired RPM of the meat racks.
Saved this for last.....
Digital F/C PID 25A SSR Thermostat Temperature Controller J S K E Thermocouple
That is a mouth full and actually describes many generic components of a process control system. This is what you would need.....
A digital PID controller - This is the "brains" of the temperature control unit. This module has a temperature probe that provides input and based on the process settings will then provide a triggering voltage to control an external device such as an electric heating element. Usually the triggering voltage is a low DC voltage that is output when the smoker or "pit" temp is below the desired set point programmed into the PID. So if you dial a PID controlled system to be set at 225* and the pit temp is 150*, the DC triggering voltage is "ON".
25A SSR - This is a 25 amp Solid State Relay, which is a switching device that takes a low voltage triggering signal to control or switch a higher voltage and higher amperage electrical circuit controlling a process (ie, it acts like a computer controlled switch turning the 110v AC or 220v AC power to the heating element ON or OFF). These come in different amperage ratings and need to be sized higher than the maximum possible amp draw of your heating element. So if you go with large wattage element you may need a SSR rated higher than 25 amps. The SSR and wiring must be sized to the load for safety.
J S K E Thermocouple - Ok, pick just one. It will be either a "J" type, "S" type, "K" type, etc..... but not a J S K E. Most PID units can be set to work with any of them and also with RTD type sensors. This is the probe that the PID uses to read the pit temperature. Any of them will work just fine for a smoker.
A note on PID units. There are a lot of inexpensive and used Chinese PID units on e-Bay for around $20. Usually these are MYPIN TA-4 models. A word of caution about those.... Sometimes that $20 can be a bargain and they work find, but other people have reported Chinese PID's that never worked properly and gave weird readings. I'm partial to Auber Instruments PID modules. The are a little more than the e-Bay specials, but Auber Instruments is in the Metro Atlanta Georgia area and they have great customer support. The Auber units are very popular with smoker builders and the home beer brewing groups. They are also referred to as "Auberins" PID controllers which is short for AUBER INStruments. Also I have seen PID units that the spec sheet shows one "pin out" and the schematic on the actual PID shows another "pin out". Always go with the "pin out" terminal identification on the actual PID if there is a schematic drawing label on the PID.
There are a lot of threads in this forum on wiring PID units in general. Main thing you need to decide on is how big of a heating element, or how many smaller elements are you going to use (and what voltage will you be running them at). From there we can fill in the blanks to give you a wiring diagram.
Clear as mud yet?