Has he encountered a problem in the transmission? Baring a significant problem with the COM clutch system, there is usually not a lot that goes wrong with the transmission and rear-end. If he has completed an engine rebuild, the biggest cost/potential risk should be behind him. The 600B series tractors have a fairly unique engine that is is getting pricey to overhaul.
My experience with these little "Rock Island" tractors has been very good. It's not hard to get upside down financially due to parts prices, so if he is doing this to get rich...he should stop now. Otherwise finish the restoration and enjoy a bulletproof little tractor for years to come.