Rebuild a Manual transmission?

Hre's the deal. My oldest son is getting ready to drive. I found a Nissan d-20 89 pickup 2wd 5 spd. Basic. In great condition. However, the synchronzer is going out in 1st gear. On a scale of 1-10, how hard is it to re-build a manual transmission on your own? I ask because it will determine if I buy it. It is a good deal but it loses it's appeal if I have to spend close to a grand to get it re-built.
Thanks
BSD

Is the 1st gear even synchronized??? Usually grinding when entering 1st (and R) but not between the other gears is a sign of incomplete clutch disengagement- bad master or slave hydraulic cylinder or clutch cable adjustment is out of spec.

Yeah, I checked an exploded view and 1st has a synchronozer. It wants to kick it out of first. You have to hold it in first with your hand. Only does it in first.
BSD

If he is mechancally inclined at all, he should be able to do it. I started rebuilding '50s vintage Ford 3-speeds at age 14. Graduated to Chevy Powerglides and Muncie 4-speeds. By 16, I was rebuilding MG/Austin Healy 4-speeds, TH-350s, and assorted other stuff. Almost cut my fingers off putting a Buick Dynaflow on a bench once.

Most transmission parts houses will have a manual for that tranny and will be able to get the parts you need. Popping out of gear is a classic symptom of bad input or output shaft bearings. Could be the synchro, or something as simple as the synchro lock plates.

I say let him go for it. Just pay attention to where the small parts came from, especially the shifter detents and lockouts if they are in the case. Worst outcome will be a truck with a bad transmission - still.

Spudboy