Jane,
The 2004 had the original LB7 Duramax engine (from 2001 thru the first half of 2004 model year) when the LLY version of the Duramax engine was ready for production. The LLY was until the LBZ version was ready for production in the early 2006 models. The Duramax engine has always been 6.6L and still is. The LLY had more HP and a lot more torque. The LLY engine vehicle are called the 2004.5 model and the 2005 model. Donn is very wrong as all Duramax engines are very good and there has to be a reason you'll want to know as to why the truck you cited had a new engine put in and for the tires, that's very hard to know as it may have been in the flood or just changed from off road style tires back to road tires. So many possibilities. Lots of these LB7 and LLY original engines now have over a million miles on them and are the OEM engines and still going strong.
Tens of thousands of vehicles/trucks in the south were in floods and you don't want one of them ever. Most were bought up as salvage and made to look great and are sold to unsuspecting purchasers. The title and VIN history along with the date on the title for the truck might be a big help. All states do not use the same designation for junked or salvage vehicles when issuing a new title so do your research with the DMV in your state. You can't tell by checking for rust on a GM as they have all exterior sheet metal made with 2 sided G-60 galvanized except for the roof. Even up here where they salt the roads in winter, you seldom see undamaged rusty GM pickups because of the galvanized coating.
BTW, our 2004.5 Chevy 2500HD Duramax CC LB 4X4 ordered new and has close to 200,000 on it now does not have even a hint of rust anywhere on it and has never needed or had any repairs done other than new front brake pads at 126,000 and 3 lights bulbs replaced. Rear brakes are original and still have a lot left. The truck is and has only been used for RV'ing since new. If we wanted a new truck, we'd simply buy one but why as ours is and has been superb and still looks new inside and out!
A superb CC LB 4X4, GM HD Diesel, airbags, Rancho's, lots more
Lance Legend TC 11' 4", loaded including 3400 PP generator and my deluxe 2' X 7' rear porch
29 ft Carriage Carri-lite 5'er - a specially built gem
A like new '07 Sunline Solaris 26' TT