First A-833 rebuild, what to look for?

All the stuff you and jeff said are must do's for sure. Have a close look at all the syncro teeth on each gear. They need to be sharp with no rounded edges. also the ring that engages the teeth on each of the gears needs to be in good condition too. If the gear synchro teeth are worn the slider ring might have wear too. The trans will pop out of gear if the synchro teeth are in poor condition. Usually 2nd gear is the worst due to missed shifts and third is next on the list. The previous owner of your trans must have been jamming it into first gear. Enjoy