Some of the ideas seem like they could be solved by off the shelf hardware. Switching loads based on temperature? Buy a cheap programmable thermostat. If you want to monitor an AC load, you can use your preferred microcontroller along with an opto-coupler. I just did this, using an opto-coupler to monitor my programmable thermostat's relay and report a logic level to a Raspberry Pi, which then logs when the relay is closed (and thus the heat running) versus open (heat off). You can get opto-couplers that include built-in rectifiers, allowing you to work with AC voltages, but of course you need to understand what you are doing to avoid danger.
I used an Electric Imp, which is a WiFi-enabled microcontroller, hooked up to a digital temperature sensor and a photoresistor, as an outdoor temperature/daylight logger. Electric Imp is a hosted solution, which is not ideal - unless someone else reverse engineers the protocol and builds their own server, when the hosted service disappears, it'll be worthless, but it was very easy to use. Here's a graph of the output.
Cost becomes an issue. WiFi connectivity is expensive. Cheapest I think you can do is about $25 - that's what an Electric IMP costs (not including a breakout board), or a Raspberry Pi A+ if you throw in a $5 WiFi USB dongle. So you're looking at a minimum of $25 for each WiFi enabled device (and neither of those are ideal - Imp is hosted and lacks much GPIO, PI is large, delicate, and lacks some basic microcontroller features). That's not very affordable, especially if you're used to throwing a $3 Atmel chip in your devices.
My thinking going forward is to couple Arduinos with relatively inexpensive RF transceivers that work in the ISM band, and simply use one WiFi device (like the Pi) as a base station that can talk to all the other devices. That will bring the cost-per-unit down to maybe $15.
Note that you will be spending a LOT of time on each project. And you will almost certainly spend far more money than you will ever save. But we do it for fun, not for efficiency!