Pfffh, i am no web dev, not one experienced with PHP or stuff programmer. But if i had such an idea in mind i would never give it a time lap of more than 2 month. The web development is quite the easy part, even if you are a beginner at it. Comeon, Phage drop websites in one nite but that is because he is experienced, i wiped up
Evilshortener in a day and i had only 2 days experience with the Flask framework or web development at that.
I say give your self a deadline, maybe one month for the site, and put up a secure prototype out there. Use push notifications for now as your mobile solution or something. Have an API and in the next month you could dub between rolling a mobile solution(app) and making you site secure and having a better UI.
The only challenge really in this site is developing fool-proof algorithms for predicting when to sale and when to buy. But if you said it is already out there, i am sure it is already documented somewhere, so read up and understand this part. You will be processing info for alot of users so you will need processing power.
So here is my project timeline.
First month, dub between reading and understanding the algorithms in the backend and developing a working acceptable secure site.
Second month, dub between improving the UI, security, fine tuning the algorithms and developing a working version for your mobile solution.
Months through this, get feedback, get disappointed at your site design, rework it from the ground up in less time (2 weeks max), pwn and secure it, network socially and technically, get more resources for you algorithms with the increasing user base, spam Xires and HTH with algorithm questions and get a set of eyes on your code. Pay them a fair lot to route to EZ servers. WIN...