As pointed out above, you don't say where in the world you are. It certainly makes a difference to the electrics (Voltage, codes, etc) and availability of parts (particularly Insulating Fire Bricks with low thermal conductivity).
Analog control is almost always obtained by using a time-proportioning output from a PID controller, switching the mains power to the elements through either a contactor (big relay) or Solid State Relay. The SSR allows shorter cycles and improved control. I use a 2-second cycle because that gave the best results during my testing with fairly limited equipment. If you have the departmental resources available for testing it, you should have no problem optimizing whatever you build. Many of us would be grateful for any information you can provide if you do.
As an aside, I have tried using a true analog output from a controller to drive a phase-angle SSR (a Crydom IIRC, probably an MCPC2525C). It did not seem to control any better and was remarkably noisy with the coiled elements. I do not recommend it.
Industrial ramp/soak controllers are available and work well, but are not usually very user-friendly, particularly when setting the program steps for ramp/soak cycles: it's not usually a problem if the builder is the user, but not ideal if the user will be someone non-technical. I found that kiln controllers available at the time (about 2010) were aimed at the ceramics market and did not allow the short switching cycles that seemed to work best for HT of steels: we tend to need tight control of temperature, where ceramics are more concerned with heat-work (time at temperature).
I'd advise researching all of this very thoroughly and suggest you bear in mind that availability of complete HT ovens at reasonable prices in the USA means that the "good HT oven at a reasonable price" sector is pretty well covered there by the manufacturers: As far as I can tell from this side of the pond, most of the US DIY builds seem to be either aimed specifically at achieving the lowest possible cost, or are built as a hobby by people whose hobbies include building things like HT ovens. There may be more useful stuff on the Australian, South African and British forums where the marketplace is different but the language is (more-or-less) the same. Wiring will obviously need to meet your local code, so bear in mind the differences.
The best resource for working out your heating element requirement is the Kanthal Handbook, freely available as a .pdf online. From experience, I'd recommend using 14 AWG Kanthal A1 if you have a big enough chamber to use it and are planning on treating stainless steels ("Kanthal" is a trade name, so you could use a generic equivalent FeCrAl alloy). How long your element needs to be depends on many things, one of which is your supply voltage and another is the diameter. I'd advise 16 AWG as an absolute minimum wire size, even if you are only intending to HT Carbon steels.
Using twisted tails to get through the walls has generally proved more reliable in the long term than than using the stainless allthread shown by Andy Gascoigne in his "heat treatment furnace" pdf, but otherwise, it is a pretty good starting point and many people around the world have built to that basic design. It's worth Googling. It is dated in terms of the controls though. Electronics have moved on a lot in the decade or so since it first appeared.
Most places that can sell you the element wire can do the calculations for surface loading, resistance, length, etc and will wind the elements to suit. I did the calculation myself then checked my result against the element suppliers calculations before ordering the wound elements. Don't forget material for the staples if you are stapling the elements into the grooves.
I filed the grooves into the IFBs with allthread for my first build. Subsequent builds have had routed grooves: faster to do and tidier once done, but very very dusty at the time.
I pretty much guarantee there is a lot you need to do, get, and learn before you need a HT oven.
