Build an SaaS web application

I want to have web app that allows users to build simple custom calculators (such as this calculator [login to view URL]) High level requirements: -https -login (email) -admin area (for site owner) -Dashboard (for user) where they can see calculators and create new ones. -calculators will be optimised for mobile -needs to be an SaaS solution so billing via stripe is needed (based on plan) In terms of style/layout something like [login to view URL] and [login to view URL] is probably the style I like but this is a much simpler system to ucalc. What users can do: -create custom single page "calculators" with certain logic -style calculator page -header & footer -Background (Image, colour, or gradient) -Title and description -Colours and size of input box, text or borders etc Eventually there will be three plans - free plan, mid plan, top plan Free Features -one calculator -social sharing links mid features -max 5 calculators -Custom domain allowed -no more "built with 'name'" branding -social sharing links -google analytics top features -unlimited calculators -Custom domain allowed -no more "built with 'name'" branding -social sharing links -google analytics The 'logic' building part should be similar to ucalc - where people add elements using UI and then link those elements with actions (formula).