Buying from Warner...
I can tell you from experience that buying a truck from Warner is a good move. I have one that we purchased from them and have had 0 problems with it. I drive a 2001 Freightliner FLD 120 that was a Warner truck.
As far as the "book" value of the trucks... it is actually hard to determain the value of each and every truck out there. With cars, pickups, suvs, vans and motorcycles there are only so many options that have to be take into consideration. With the trucks, as you know, you pay for pretty... and the engine/trans/rears setup. The best thing that you can do is find the truck you are looking for and search "truckpaper.com" to do a comparison of similar trucks and the price they are charging for them... but you want to look at the retail sale ads and not so much the personal. Private truck sales seem to be a little higher because people are trying to get some of the money back that they poured into the truck. So, you can buy a 3 y/o FLD120 w/ 400k - 500k from Warner for about $40k or so... the owner operator is sure his truck is worth $55k. Especially since he just paid $14,000 for the engine in it with only 200k on it and 5 months before that he paid $8000 for the transmission that he blew apart.
Just what ever you do remember that even though they take care of their trucks pretty well, there are going to be a few with issues with some of them and you need to take your time to inspect the trucks very well before you purchase. Best of luck to ya!