Web Site/Service Error Monitoring

I am currently working on a project that consists in creating a service that will be used to better monitor some web sites and services that my company offers.

The service will need to:

• Show information about the status of the website/web service (through an interface perhaps). Ideally, it would show the status as 'Green/Orange/Red' depending on the actual status of the website/service
• Have a mobile-compatible counter part that can be used to access the service from anywhere. This would probably have the same interface mentioned above, adapted for mobile devices (iOS and Android)
• Be able to get error messages (email in most cases) generated from the already existing solutions, and use that information to 'determine' the status of the site/service

The service will be hosted onto a server in a separate location from my office (we'll buy/rent that)

I think its useful to say that I am a junior developer (so you can get an idea of my experience), and that I have already been doing research about this matter. I have read that there are some good pieces of software out there that can help with the monitoring of the websites/services and can even automate the testing process for those (the best I found for my needs are Selenium and Nagios).

I understand this question may be considered as vague, but I have tried to make as specific as possible. Moreover, I think this would be a nice topic to discuss on StackOverflow and might be quite helpful to the community, especially for someone like me that is just starting working on websites and web services. I am looking to get some tips about anything that relates to this problem, like already existing software (open source would be ideal), frameworks to use, and anything along those lines.

Thanks in advance for any help!