IMHO... you'd be better served by stating:
Why you need another team member?
What skill/skills are you looking for? Or this new team member should possess?
You could also cut down on costs if you went with some ESP modules instead of the wi-fi shield.. (and possibly get rid of the Arduino's too)
I'm not clear why anyone would want to join the team for free?
Arent you already at the hobby level? So the next is commercial? Or is you dont have a working hobby version?
Also.. I'm not sure your current approach.. but if things need to change/update in real time.. how are you sensors and stuff all communicating?
You might be better served using a RaspberryPi 3 set up as BOTH a:
MQTT broker (for all connected devices, you no longer need to POLL a database for changes.. MQTT is a subscribe/publish low overhead type of platform)
Install LAMP (web server, mysql, php, apache..etc) on the RaspberryPi as well.. this way you can also log data, as well push out notifications to the correct device.
(There are several PHP classes out there to communicate to MQTT server)
RPi3 with MQTT/LAMP install
- Arduino #1 with temp sensor connected.
- every xx seconds.. you send, save, log the temp in the room
- you can have logic that if 'temp' gets to be above your desired threshold, that it immediately, not only logs the data, but pushes out notification to the 'Fan' (connected via Arduino or ESP..etc) that is subscribed to the topic instantly.. and the fan is on.. the "FAN" (device) doesnt have to do anything but sit there.. and wait for the notification
IMHO.. this is the best approach for your project..