First of all, thanks a lot!
The job, anyway, has been done by a mechanic and I have no info about a few asked things. I will let you know soon.
You gave me many good info, and now I feel more comfortable with this situation. Soon, I will talk with my mechanic, and we will decide what to do.
I think that no one took care the transmission lately, probably many thousand km ago. About the money I paid, when I went to get the Insurance with RACQ, the lady told me that this car worths 2.700AUD, following the market. So, I thing, if I paid 2.000AUD, it is not bad, especially because the body and inside as well, are really well maintained. No leaking at all and the engine sounds good. The only mad thing is this transmission.
I will bring you up to date, thanks a lot for now, I appreciated all your advises.