Sage50: I’m not sure how it works from there to export master data like Customer, Vendors, Item, COA etc… But surely you could import excel file into ERPNext using Data Import tool.
For the template of Data Import of ERPNext, make one dummy entry into ERPNext and then
use the ‘update record’ option >> Download template with existing data >> use the dummy data format to capture your master data from Sage50 >> Upload it into ERPNext >> If there is a conflict or error reported, change the template data and re-upload it.