Review and revise an Excel spreadsheet

General information for the business: Import and distribute outdoor structures for the hospitality industry
Description of excel work: I have an existing spreadsheet that was designed by our Italian partners that is difficult to manipulate and won't work in G Suite. The sheet is aimed at restaurants, cafes and bars and hotels. The spreadsheet enables one to work out the following:
AREA DEFINITION
It defines a sqm area by inputting width/projection
It defines number of tables by inputting a sqm per table
Number of seats per table
Average occupied seats

PROFITABILITY
Ave expenditure per seat
% margin per seat
Net profit per seat
Operating hours per day
Table rotation per day
outputs - Ave daily turnover and Net daily profit

PROJECT COSTS
Product cost + associated costs
Outputs - Cost of total project and cost per sqm

ROI
Output - Necessary days to reach payback (I assume this means how many days either turnover or profit to pay for project)
Ave of extra necessary days? (I assume this means lost days due to weather etc.)
Recovered days on a year basis (I have no idea what this means)
Closure for holidays
Outputs - extra generated turnover and extra generated profit
Output - return investment yearly basis (I'm not 100% clear what this means)

I need to add the following functionality

FINANCE
I need it to read the total cost of project
I need to input cost per thousand (currently £39.50)
And it needs to output a monthly cost which will be for example:
Project cost - £23,500
Finance cost = £39.50 x 23.5
I also want to be able to change month numbers to either 12/24/36/48/60 with a different cost per thousand that I can apply for each variation.

What I need is the existing sheet to be reviewed, areas I don't understand made clear, sheet to be labelled correctly, enabled to work in Office, G Suite (Google Sheets) and Libre Office plus the finance functionality added. And lastly I need it to look nice because clients may want to look over then salesman's shoulder.
Original is attached
Extra notes: Needless to say this spreadsheet is confidential and not to be shared with any third parties.