Hi Riccardo and welcome! I bought a 1979 Dufour 31 4 years ago and almost finished refurbishing it… the owner’s manual , such as it is, came with the boat. I have written to Dufour twice for some info, but never had a reply.
Regarding the keel, the important length is thedepth from the waterline, so make sure you are measuring that.
Regarding antifouling, if you have money, go for copper coating, which will be a few thousand but will last for years. If not, rub down with sandpaper and then ise 2 or 3 coats of antifouling- I ise International 250. Silic One is good but you need an epoxy primer first. Last winter I got the hill sandblasted to get the old antifoul off, then used a primer and 2 coats antifoul. This is worth doing if it’s obviously irregular and patchy. Hope this helps
The membership association for owners of Dufour Yachts