how hard is it to rebuild one im pritty good at machnical work but never done one, iv done more or less everything else but have't atemted a box

the thing is i ran it low on box oil and it wooing in every gear, i think the bearings are on there way out it only does it when the clutch isn't pressed

and the other thing it when it's cold the gears are stiff ish like the oil is thick syrup but its fine soon as iv done a mile or 2, any idea what that could be?

any advice would be great

thanks chris