My guess is 40kg. Remember that you will be on your back or your chest so all the strength to manipulate the transmission will have to come from your arms. If you're working alone you will need a transmission jack or at least a floor jack. Even if you're not working alone it is the safest way to remove and install a transmission. Figure that the floor of the car has to be high enough for the height of the transmission and jack. If you use a regular floor jack, also the height of a wood block so the jack saddle doesn't damage the transmission pan.

Sixto