If I understand the requirements correctly, that is a domestic water consumption monitoring system with internet visibility (cloud service) and e-mail alarming etc. I'd probably use an architecture with a relatively simple part locally and most of the functionality in the cloud service. It is not recommended to run a web server locally which has to be visible to the internet because this often requires a complex router configuration.
As a concrete example, an ESP8266 (as has already been suggested) and a water flow meter which can tolerate the mains pressure at the OP's installation.
In the simplest case, the WLAN credentials are embedded in the sketch. This would post the usage data, say every 5 minutes, to an internet based (say) LAMP server. The LAMP server would maintain a table with flow rates with timestamps and run two jobs, one to send an email if a consumption threshold has been exceeded and one to send an email if there has been no data received within a timeout period (power failure case). That can also be kept quite simple if the basic admin features (direct table manipulation) of the LAMP server are used to maintain data like alarm receiver email address, alarm thresholds etc. instead of creating a lot of custom html.
I don't generally do this sort of paid work and am not going to get further involved in this activity, but I have done something similar, which I published here, for my telephone installation at home, that is collecting data and making it available on an internet based LAMP server. The internet service I use, incidentally, which includes the LAMP server, domain etc. costs the equivalent of about $100 / year but this can also be significantly cheaper.