Re: 3rd and 4th gear Problem
1/ If you go down the route of the gearbox exchange then why not look at a 2nd hand one? I got one for £140 plus transport. I fitted it myself - we on this forum can always advise you on that part. In a garage it should be a 6 hour affair including the clutch change.
2/ On the web I saw one in UK for £175 plus transport. For that price I think it comes with a short-term warranty.
3/ Anyway, just make sure it's done low mileage. Mine goes well.