I don't have any of the threads bookmarked. A
Google search should net some results to start with.
Like I said, monitoring is no risk at all. When you start controlling things like heaters and sump top-off is where the risk comes in. You have to decide how much risk you can live with. PCs are less reliable than something like a PLC. There is a lot more going on within the PC OS and hardware. Sure, a PC can run for months without a crash or reboot, especially if it's not doing anything. What happens if the OS crashes while the heater is on, or the sump is being topped off? Or the controller process itself dies? The more you control, the more things can go wrong. Basically it's all about risk management.
You can alleviate some risk by having fail safes outside the controller, like a heater set to 2 degrees higher than your set point plugged into the controller and a second heater not controlled by the controller set a couple degrees lower than your set point. A high level float switch could shut off your auto top-off in case the controller doesn't shut it down. Lights aren't so much of a big deal if they are left on or off for a day or so, but if you are on vacation for a week or so it could be. If the controller does die, how would you know?
Whatever you do, you'd have to think about failures and what effect they would have. Even if you don't have a controller and add water to the sump manually, one failure could be one's memory.
Building a controller is more of a hobby project than a way to save a few bucks. Of course in the end you'd have a controller that does exactly what you want, assuming it works the way you intended.