IF array with COUNTIF??????

Ok, i have an overhead table in a sheet (attached), im going to use dependant dropdown lists for category & sub-category. The next field is receipt number (if applicable) The reason is we assign daily expenses (carpark ticket, fuel, train etc etc) with a receipt number 1,2,3,4 ascending as high as necessary until the next business year when we start a new BLANK workbook template. However we do not assign "overhead" items a number as these are generally Monthly or annual receipts that are just stored in an overhead folder. However i want to record both Overheads and expenses on the same sheet.

So, what i want to achieve is for the receipt number column to automatically generate either a receipt number (which is +1 to the previous receipt number) or enter the value"Overhead folder" Ive got as far as =IF(D2="Overhead","Overhead Folder",IF(D2=Expense,SOME SORT OF COUNT FUNCTION,""))

My logic tells me i need a function to count the number (not test for the SUM of the cells) of cells in the receipt number column which contains a number value only and return a value to the cell with +1 sort of like an auto-number feature but as im fairly new to the advanced stuff in excel im not sure how to express it.

ReceiptNoForumHelp.xlsx
You might of guessed, i want it to return as blank if neither are true.

Any help greatly appreciated.