I asume this will be a manual transmission?
Any leaks from under the car at all? considering you mention filling the gearbox?
There are no leaks from under car, the oil is at the required level.What else do you think is wrong?
Ok..from what you describe i dont think the gearbox is at fault - but more with the clutch itself could be slipping especially so when under load going up hill - of the revs dont match the speed of the car ie: revs are way higher - but no power to the road - if thats the case then a new clutch assembly needs to be fitted -