micra K11 making a whining sound when in 1st & 2nd gear??

hi guys

Ok so I just took out all the fuel injectors and cleaned them all and we now have lift off as one was not working ..so I was able to fix that myself ..

Now I still have this whining sound when it is in 1st and 2nd gear , the service and repair manual says it could be
Worn, damaged or chipped gear teeth ..
so now I want to ask
1- should I just buy a 2nd hand transmission or a new one if they still make them ??

2- or fix the one that is in it ??

3- could I do this myself ?

4- if I don't do it how much would it cost ??
5- has anyone had to do it to there car?

thanks guys

Victoria,

I tagged you in a post on Facebook.
A guy on there may have a gearbox but he's near Newcastle.

John