What is Luchresi's role in the story? And also, what preparations had Montressor made for his revenge?

Luchesi is the trump card, the ace up the sleeve that Montresor uses to dupe Fortunato into the catacombs beneath his home. Luchesi is a second rate rival of Fortunato's in wine expertise. Montresor has bought a keg of Amontillado, a rare and pricey wine, which Luchesi assures him is the real thing. Appealing to Fortunato's pride in wine connoisseurship, Montresor asks for a second opinion. Fortunato's pride is stung by Montresor asking Luchesi's opinion first.

The rocks and mortar were already in the cellar, as were the chains. Montresor carries a trowel hidden in his cloak. Getting Fortunato drunk, exposing him to the niter on the walls, which made him cough, renders his resistance weak. The surprise of the attack seals Fortunato's fate.