I'm thinking that you don't particularly need to monitor the fans themselves (as that sounds hard), but do need to monitor temperature and send an alarm.
Although I have not used it, a friend is a big fan of MQTT (http://mqtt.org/). This is an internet of things messaging broker. He did a nice demo of a long strip of neopixels, then in a techtalk at work, each person who visited a site with a mobile phone got control of one pixel. MQTT was the bit in the middle.
It sounds like you need to monitor the temperature and humidity (BME280 or DHT11), send to a data logging service for graphing (graphs are always good to look at), and if a threshold is hit MQTT can be used to send a message to an alerting system.
dave-in-nj suggested an ESP8266. I really like the Wemos D1 mini which is pretty much a small arduino with the ESP8266 wireless chip. You could have one in each greenhouse doing the temperature logging, then use MQTT to trigger alerts via mail and/or an alert box in your store. With that you could get very inventive.
Use another D1 Mini for your in-store box, but look at some of the outputs you can get. A buzzer and silence button is one option, but with the likes of Neopixels (or WS2811 LEDs, similar just not Adafruit branded) you could scale the colours getting redder the hotter it is and flash if an alert is triggered. These come as strips, matrix and rings. You could show a pattern or basic message on a matrix. LED matrix is also another consideration.
If you really got carried away, put another couple of sensor probes about, one in your store and one outside then you could have a display customers could see.