The Avalanche with the 5.3 Vortec will be very reliable and should make it well over 250,000 miles before it starts giving you any major issues. You can expect around 16-18mpg on the hwy stock. My buddy had a Silverado 1500 with the 5.3L and never got over 17mpg stock. He gave it a Magnaflow muffler and and K&N air filter got 19-20 hwy mpg almost every time. I think the interiors are great, especially the LTs with leather and all that. I like the newer 2006 style interior better than the 2002s, with the 3 mode heated seats, Bose sounds system, rear radio controls, steering wheels controls and what not. Overall, I think they are great trucks in which you will not be disappointed!