PNW Redneck

Im looking to trade my ranger for an 02 f250 7.3 6 spd. it has the ZF S6-650 6 Speed 4WD trans. it currently does not have 3rd and 4th. couple questions. A. How hard is it to rebuild these? B. How hard is it to find a good one without ordering a reman online? im heavily considering both options but want a second opinion from people that are far more expierenced with fords than me. im willing to do the work to rebuild it but im also just fine with swapping it.