There's two types of the cheap small arduino clones about. There's the Nano and the Pro Mini.
I've got some of both but I much prefer the Nano type. They come with USB serial built in and possibly more pinouts, slightly larger and marginally more expensive I think. You can power over the USB connection too which is really useful because I've got a load of mini-USB phone chargers from old phones lying around. I believe you'll need a Nano for the nRF24 adapters because the Pro Minis I have don't have a 3.3V pin which is required.
If I've still got a spare Nano I might make another wireless temperature sensor today for a different room.
There doesn't seem to be a huge difference in price between the cheapest minis and nanos. The USB socket does sound useful though. Some of the mini boards say switchable between 3.3V and 5V.
The minis require you to have a serial adapter of some sort to load sketches to it. Once loaded you don't need the serial adapter connected anymore but I found this to be a pain to get working first time every time. The USB nanos work first time, much less hassle.
I have 4 in use and I think I bought 5 nanos and 5 pro minis a while back.
That does sound easier. I'm sure I have a few mini USB leads around too. Just over £2 for an unsoldered board. I might order a couple of them and wireless modules and maybe some temp sensors too. If I had more time I could get carried away with these.
Had a quick look on Google. Depends how much current the board will be consuming. If it's not doing too much it should be ok with the 18V, otherwise it would need another regulator that could handle tthe current.
The Pro Minis (without USB) need a USB to Serial (FT232) adapter of some sort to program them.
Because my adapter doesn't have the correct reset pin or something like that it's a pain to use, I've got to reset the board at exactly the right point to get sketches to load.
A little graph of the temp monitoring using a LM35. The temp at the start was in my bedroom, dropped when I brought the laptop into the kitchen. Went back up with the oven on. The drop down to 6°C ish was when I put two frozen peas either side of the sensor.
I have quite a few things on the way from China <twiddles thumbs>
bastard by name