I already contacted the owner, I have an agreement to buy for 70 euros
Sounds like it's a done deal. Suggestions of other computers are a waste of print.
spend a lot of time learning how to set it up and interpret its recommendations. Referencing your computer while you dive will give you an excellent education on your body’s ability to absorb and shed nitrogen.
I agree that you should study your computer and know all of it's functions very well BEFORE you go diving with it so that when you are underwater, you don't have to think about which button to push or what the computer is telling you. You will learn more about your body's relationship to the data on the computer over time.
Good luck and happy diving!
Cheers -