Is there a multipurpose cooking appliance that can be used to make natto?
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.