I made a project pretty like your, but for hydroponics, we control eC and pH, with schedule, including relays, humidity and light sensors, etc etc... at real time and supporting IoT protocols.
and indeed I start with a mega2650, worst desition ever, the chip is old and there are much better and cheaper around, try to get a new one (ESP8266, CC3200, CC3200MOD, ESP32), no need to be the best, just one with good support and not complicated to mount for prototyping, if I would have to make that project again I'll choose a wifi chip only for communications and others for control. Examples and support are important to code fast, so probably TI chips are the best option for this.
Now, as advice, start simple, simple as possible, for example, take a Rasberry pi 3, buy a CC2650Sensor Tag for humidity, temperature, and other ambient variables you are interested and integrate it.
then make your own Relay board, is easy and cheap. Don't try to control big currents because when you has to put price to your device (with all the protections you MUST have) you will realize that when you have 1KW of power is better to add a contactor switch controlled by your relay (if you have the money for 1KW you have the money for the contactor switch), so, use your relay to control up to 400W of power and/or to control that contactor switch (all at 110/220V).
So far that is your best and cheap way (for me) to get your system working at a shorter time.
If you have doubts feel free to contact me.
Regards,
JP