newbie with gearbox questions..

[jbh110tdi]

Hi there I signed up after buying the wife a mk5 1.4 tdi to use as her daily.
so this is my first post and sadly to say ive got gearbox questions...

coming up the box everything is fine but coming down the box it won't go into 3rd..
If I go from 4th to 2nd then to 3rd it will go in.
or if I release the clutch blip the throttle then dip the clutch it will go into 3rd..

now ive read of other folk having the same issue so not sure a second hand box out the breakers is a good thing as I would be wanting a known good box..
so ive read of folk doing conversions is there anything I can do on this to use a different box?
Pretty sure I have the hydraulic clutch already with it being the tdi and its a cable change.
so would a mk4 golf box fit?

cheers
James