For what you're planning, one of the smaller heat treat ovens should work just fine. I originally used a Neycraft furnace that I bought from Brownells. It had a chamber roughly 9" square, which was more than adequate for small parts like locks. It had an analog control, which worked well enough—though most furnaces available today are programmable, which is a nice feature to have (though not absolutely necessary).
In terms of temperature range, most small ovens will go a bit over 2100°F, which is more than sufficient. That said, keep in mind that running at the upper temperature limits, especially when doing carburizing (due to the gases produced by charcoal), can significantly shorten the life of the heating elements.
These days I use a large Paragon furnace with a Sentry controller, which is excellent—but expensive. If I were in your position (unless budget isn’t a concern), I’d recommend looking for a good used oven from a reputable brand. They show up occasionally at pretty reasonable prices. An older analog model may actually be easier to operate and repair when something eventually fails (which it will).
One important consideration is power requirements. While many smaller ovens run on 120V, they often draw high amperage—typically 12 to 15 amps—which can overload a standard household circuit unless it's on a dedicated line. If you have access to a 220V outlet, a 220V model is generally a better choice. It will heat up faster, run more efficiently, and is less likely to trip breakers, especially during longer or higher-temperature cycles.
If you're looking to buy new, there are some more affordable options on Amazon. Vevor, for example, offers a small model around the $500 range that looks decent for occasional use.
Lastly, here’s a link to an old thread I started about 15 years ago on color case hardening. It includes photos of the original Neycraft furnace and setup I was using at the time:
https://www.homemodelenginemachinist.com/threads/color-casehardening-a-photo-and-video-tutorial.4520/
