Budget Calculator MVP

General information for the website: Minimal Viable Product (MVP)
Kind of development: New website from scratch
Num. of web pages/modules: 1
Description of every page/module: I am working on a idea for a budget calculator where a user would be able to name their list, provide a budget amount then start adding items to be deducted from that budget. For example if the user wanted to create a Wedding Budget they could call it 'Wedding', place a budget of '£15,000' then add items such as 'Insurance', 'Wedding Cake', etc and the cost for each item. Once they have finished listing their items (Which they would also have the ability to remove too) they would see how much money was left from the budget.

As an MVP (Minimal Viable Product) the main feature would be to calculate but with the possibility to download that current budget list (e.g. could be as a PDF or .xlsx file) and Send via email to themselves.

The single page application would not have a login (this is just an MVP at this stage) but as anyone could access the page then there would need to be ID sessions in place. So if I was to use the budget planner, then someone else could use it at the same time without overwriting or seeing what the other person has done.
Description of requirements/features: * Ability to add text fields (strings and integers)
* Download (PDF, .xlsx, etc)
* Send via email
* Save option - could this be possible without the user creating an account but with cookie sessions?
Extra notes: Example URL's:
http://personal.natwest.com/personal/savings/tools-for-savings/budget-calculator.html
https://www.ucas.com/ucas/undergraduate/finance-and-support/budget-calculator
https://www.moneyadviceservice.org.uk/en/tools/budget-planner/short-form/edit/money-in

The links above are more specific e.g. Monthly/Household and Student finances. But the planner I would like to build would allow any type of budget.

I am open to suggestions and thoughts on my budget calculator.