I'd like to suggest you invest in a manual for the tractor. If you search the net, you may find one for free in PDF format or you can buy one for under $40.
I don't think the tractor cares too much on the type you get. Probably any TDH (Transmission, differential & hydraulics) type would work. If you go to the tractor parts store, they may have a book that recommends a particular type.
On those older tractors (and why I recommend the manual), it's common for one tank/reservoir to supply the tranny as well as the hydraulics. Instinct may say to fill the reservoir but look closely and you may find a dipstick which lets you know the fill level.