Any WiFi network you like. I was assuming for some reason that you wanted a standalone solution, e.g. for an installation. If using an existing network you don't need to configure a Pi as an access point or DHCP server.
Here's an example of a connected greenhouse we built using MQTT for integration.
The sensors, fan vent motor, fan and horticultural LED module were connected to one system (an Intel Edison, but a Pi could be used instead). Then a 2nd system read inputs from switches and drove LED displays. They both communicated via a "cloud" system (a little PC running Linux, Mosquitto and Node-RED). The logic was implemented in Node-RED, which would subscribe and public to topics. The code is at:
We later added a "bridge" that was built using a Raspberry Pi. This republished messages to/from topics on the broker which ran on the local cloud PC, via a 2nd connection to an instance of Mosquitto plus Node-RED on the Internet. This was used to provide a graphical dashboard online, with temp and humidity etc. data. Doing it this way meant that if you lost the Internet connection (it was all installed inside a truck touring the UK) it would carry on working, via the local cloud.
Writing Python code to MQTT pub/sub is dead easy.
A quick search turned up a few Android MQTT clients you could use, e.g.:]
There seem to be code examples too, so you could make a custom app. But also if you use Node-RED for the logic, there are tons of other interfaces available too. So for the control/user interface side you could drive it all via e.g. a web page, a database, Twitter etc. You can browser the available nodes (e.g. input, output and function) and flows (a Node-RED app is called a "flow") via: