Well as someone who has done both i.e. taken a gearbox off and replaced the thrust washer refitted and had the box fail ( not at all sure why - maybe something internal got damaged) and also then replacing with a known good gearbox, I would say that I would go for replacement.
I had two significant adavantages.
1) the help of another( strong) forum member who claims to be my age but is actually only 22.
2) I built a level ramp to drive the whole car onto that is 40 cm high. Thathas proved a boon for working on the car as I have had endless exhaust problems.
My recommendation would be change it for another. If yours is a 2.5 then consider an AR35.
You definitely need two people and one or two good trolley jacks. The job itself is fairly straightforward. disconnect propshaft, disconnect about a dozen bellhousing bolts, disconnect wiring loom plugs from gearbox, disconnect wiring on cats and remove both cats (mine had been off times so came off easily). The key thing is to remove horizonatally and refit horizonatally to avoid damage to the engine output shaft. I think Haynes manual gives quite a good amount of info.
Hope that helps.