

Replacement is usually removing 6-10 screws and prying the case with a guitar pick or old credit card. There is most likely a disassembly video on youtube. Batteries from aliexpress or the like are usually cheap (although probably more expensive than the computer). Depending on the application, the “built-in UPS” can be nice.
I do SMB support, so I have a pretty good idea of what people tend to do.
I haven’t seen a PS brick catch fire (possible, OFC, but extremely rare in my opinion) i have seen a PC PSU catch fire, and because of the fan, it’s fucking scsry, like a jet with the afterburner.