Who did the refurbishing? Was it manufacturer refurbished or a re-seller?
This was not refurbished by the original manufacturer.
Is the battery made by the manufacturer or is it an aftermarket battery?
Because these items are refurbished I cannot say with certainty if your order will contain a manufacturer original battery or not. Each item will have undergone various degrees of renovation.
Can it be upgraded to the current mac os version?
I just received mine yesterday and upgraded to El Capitan without any problems.