The total value of the Villa is 150,000f, so 80% is 120,000f. Buying all the buildings and all the building upgrades gets you close to 100,000f. The remaining 50,000f or so in the Villa's value comes from buying armor/ weapons and collecting. If you get all the building upgrades and buy all the weapons and armor that appears in the shops, then by the end of the game you'll be almost garenteed to have the 80% needed, if you also collect all 100 feathers for the achievement, they will also contribute significantly. Other collectables such as the codex pages and assassins seals will help as well. Being story-related they are hard to miss.