What to replace-A833 rebuild

The countergear shaft is junk, and the bearings will be untrustworthy as well.
The blocking rings are junk on 3-4, and the hub probably needs a nice wire brush run over it, but they are nearly impossible to hurt.
Can't tell by the pic, but... if the 3-4 blocking rings are shot I'd guess the 1-2 ones are as well. 3rd gets the most abuse next to 2nd, so I'd replace them. Probably stretched as well.
That 3rd gear doesn't look too good. I'd shelve it.
Reverse is pretty rough as well. You can make a pretty bad reverse work, but it will likely start grinding instead of meshing soon.
Shift forks should be measured.
Keys/springs should always be replaced.
That is just a very worn out tranny. No real signs of abuse, but it will need a major overhaul. Looks like pretty high mileage.