Build a website which follows price evalution of a product and sends alerts upon user defined trigger

I want to have a website on which user can create a "watcher" by pasting a link from other sites (GearBest, Banggood, Geekbyuing, Aliexpress, Alibaba). A preview of the site should be shown where the user should be able to select the area or part which shows the product price. (alternatively, the system can identify it automatically if recognizes the page structure) The watchers are like monitoring tasks. They have to be configurable by time interval ... every 60 seconds, 180 seconds ... 5min, 15min, 30min, 1, 3. The "watchers" should have states - draft, active, non-active. Only the active should be actually pulling information from the other sites. Each user should have a profile where he can see, update, create or delete his "watchers". Each user should be able to have at least 100 active watching items simultaneously. The users should be authenticated and his watchers to be stored in his profile. Authentication to be via one of the Gmail, Twitter, Facebook account. Within the proposals, I want to receive an analysis of the feasibility and server load which will be required to support the solution you are proposing. What kind of CPU, RAM, OS etc you would recommend.