Hello,
I'm very familiar with similar types of projects, and have done many of them.
My opinion is that you can go few ways with this:
1. Your excel is the data source (like a database), this could be one good option, because when you change something, it will automatically be implemented, refreshed. There are some drawbacks, but they could be solved for example with git very nicely.
2. Backend script reads this excel file and uses it, similar to 1. Maybe an additional layer would be added, but probably this way it would be more predictable customisable and controllable, but probably not simpler.
3. Frontend does all calculation based on your excel (this would work unbelievably fast, I can demonstrate). Also, don't worry about someone stealing your payroll solution, there are ways to protect this.
This would be most expensive, but would work like magic, instantly.
Questions:
1. What does user input (all people, or is it for one person, working hours, overtime, etc.?) and gets payroll for one or all persons?
2. Will you use same excel for all clients, or just one client? For example, more companies?
3. Is there something specific to client in the excel?
4. Can you send at least a part of this excel to have a look?
I propose 2000.00 EUR.
Let's discuss details, and I can give you revised offer. We would easily agree on milestones.
Good luck!