A lot easier going from Excel to InDesign. That said if this is just a table, select it in InDesign and the try to copy/paste it to Excel. Should give you a reasonable starting point but identical is not likely to happen.
Excel can and will reflow cells at whim, so exact is not even a valid concept for normal Excel cells. Consider making two graphics (top and bottom) and putting them in cells, but really Excel is not where one has letterheads. One has them in Word (with similar but not identical difficulties).
In Excel you can set header and footer in the print page. IMHO the easy way is to put an image as center header and one as center footer setting the height of the page accordingly.
Maybe this could be set up as a form saved as a PDF that your client could fill out in Acrobat Reader. The whole center section could be one large wrap-able text field.
This would make his fillable version exactly the same as the InDesign one.
The reason i need it to be an Excel template is that the client wants to produce his Invoices in Excel, using the 'math' functions as he types up invoices.
This is how the blank template looks, it becomes and Invoice or Letterhead depending on the info he inputs (but the framework remains the same):
This is why it needs to start off as an Excel file. If we have blank templates printed up onto a nice stock... the letter or invoice info he types will need to land in a consistent place on the page when he prints it from a home desktop onto the headed paper.
I'm making good progress with the layout in Excel so have hopefully nailed it. Basically created an A4 grid in Excel using cells and added the desired borders and fields to it so it matches the (In)design version he approved.