I bought a used 800s with 580 km. it was mint and the 2nd time I rode it the tranny piled up. 3k for a new one. I searched for a month to Find a used one and had no luck. Your probably going to have the same problem. Also with the Trans being so weak I would recommend getting a new one. If someone is parting out a rzr it was probably abused and you could end up with another bad gearbox failure.
Off the top of my head I don't totally remember. I believe the entire left rear side comes off. Cv's A arms. Rear diff. Exhaust. You have to pull the motor all at once in order to get at the tranny. There's no way to get it out without pulling the motor