WordPress plugin calculate fee quote for property purchase/sale

A wordPress app to allow Solicitors, Conveyancers, Estate Agents or Mortgage Brokers to give an estimate of costs for conveyancing which includes legal fees & expenses, Stamp Duty Land Tax (SDLT), Land Registry registration fees and Land Registry search fees.

I have drafted the html pages for the backend and the front end but require assistance with:

The JavaScript to calculate the tax liabilities for purchase/sale, leasehold/freehold properties. Currently I am using excel to do the calculations and can pass the excel file to you but I have set out in the design brief what the calculations should be for each transaction type.

Making this a paid for app by yearly subscription. This will need to detect when the business installed the app and disable it after 365 days. I am not sure of my options here so will need advice. I have a UK bank account but need advice on how to take payments.

Where to host the plugin and how to create updates and deploy them. I am a newbie with JavaScript but my CSS and HTML are ok. If the Javascript file can be notated I will be able to navigate and do many of the updates myself when required. This may be because of a change in the law on UK property taxes.