Is there a multipurpose cooking appliance that can be used to make natto? I want to try making some of my own natto, since it is a bit pricey getting it from the store.

Looking at online recipes (such as this and this) it appears that at one point it is necessary to maintain the mix at slightly over body temperature (100-115 F) for about 24 hours. I'm not sure if I have the equipment for this at home.

My oven's dial goes from 200 to 550, although there is a "warm" setting for gentle warmth. I'm not sure what temperature the "warm" actually is, but it seems like something between 50-80 C. Also this oven is gas-powered, and isn't too great at keeping temperature stable, in the past I've had it deviate by 25 or even 50 degrees (especially when trying to reach 500). My oven thermometer is cheap and low quality as well, so I'm not confident about maintaining proper fermentation conditions for natto in the oven - but perhaps I'm being too cautious.

Nattodad on Youtube shows in one of his videos a simple device he built by wiring together a hot plate and thermostat. I've actually constructed similar circuits before, but DIY-wiring electrical devices makes me nervous due to the potential fire hazard - although I could set up a special area in my house to minimize the danger. The other issue is that I don't happen to have a hot plate or thermostat on hand, I would have to spend time and money to obtain them.

It seems to me that I'd be much better off getting some general purpose cooking appliance capable of maintaining low temperatures long-term, that is useful for things besides making natto. That way I'm not spending a bunch of money on something that can only make natto. For instance, this video apparently uses a programmable pressure cooker. That sounds great, since a pressure cooker is pretty useful - but it's not clear to me how exactly this works for natto.

What are my options here? What is a practical way of setting up the natto fermentation temperature? Is there a way to do it without buying a dedicated hot plate with thermostat?
A programmable pressure cooker only works for natto if it has a natto specific setting. Then it doesn't turn on the pressure, or heat up to high temperatures, but keeps the needed temperature range over the specified time. 

If you want to buy a pressure cooker, look for a model which has a natto setting - consult the user manual if you need to, since I don't think it is listed very prominently on feature comparisons. It can be hidden as part of some other function, I think mine lists it as a subset of the yogurt making function, and then the button which usually chooses between high and low pressure in the standard programm chooses between a yogurt (43C) and natto (35C) setting. 

Getting a programmable pressure cooker is a good choice in principle, since it is indeed a useful tool. While all the things it does (except some niche functions like that natto) are also doable on stovetop, it does these things with more convenience and less energy expenditure than the stovetop. So, if you pick a model with a natto function, it should hit your other criteria of usefulness too. 

If you prefer the DIY solution, you don't have to solder your own temperature controller. There are inexpensive thermostats with sufficient precision for home use. I switched my yogurt making from the Instant Pot (the programmable pressure cooker mentioned) to a ITC-308 controlling a cheap water boiling stick in a gastronorm tub, with an aquarium pump for added circulation. The combination is somewhat cheaper than a prepackaged noname sous vide stick, and gives you more flexibility in the temperature range, also you could even use it somewhere else around the house if needed. I think the accuracy might be slightly lower than that of a good quality sous vide apparatus, but it is sufficient for the sous vide preparations I have tried so far, and will certainly do for a traditional process such as natto.