Build Me A Site Uptime Checker -- Reposting

I'm looking to have a tool built like [login to view URL] I'd want it to run off our Wordpress site (i.e. able to enter the info from our domain) but the actual work should be done on an Amazon instance so as not to bog down our site servers. The big difference that I'm looking for between what I want to build vs what's there is that I don't want to really monitor for changes per-se I want it to be more a check for JavaScript sites so I need it to be able to crawl as one of the known bots (ex - Googlebot) and check for specific content. Basically, this is a checker for failing prerendering systems on JavaScript sites. If prerendering fails, some sites I work with send a 200 but just the default page so we can't monitor for 404s. To get around that I want the user to be able to enter HTML that would only be on a page if the prerendering system is functioning properly which will generally only be served to bots and setup frequent scans for that content. If the user has to enter the HTML they want checked for that's great. A step forward that could wait until future iterations would be to have the page render as the bot to the user and allow them to select a page element they want monitored rather than entering HTML. Basically - allow non-tech business owners to setup monitors. If it can also function as a content checker like the example tool that's great but the core function is an alert system. People should be able to create usernames and passwords to they can store different alerts and set different agents and frequencies. For right now it will be a free tool with an interval of hourly, daily or weekly checks. I will want to increase that to every minute or 5 minutes down the road and also full crawls of sites looking for errors and timeouts but that can wait for v2. Additionally - for right now we'll just go with email alerts but down the road I'll probably want to add text.