We had an 03 Excursion with the 330 hp. 3126 model. Absolutely no problems with the Allison or Cat. Lots of power. Hopefully the coach has good records and all preventative msintance is updated. Ours had been sitting for almost a year and not been serviced. The cost of oil change, tranny filters along with Allision transmission extended life fluid and many small neglected items added up. However, that's what one gets when one buys used! You will see a great number of Discovery's on the road.
We enjoyed our 03 and I am sure you will also. Have an independent inspector go over the coach. Money extremely well spent! I agree with gauto, $60 80k is the right price range. Check the tires, cost is high to replace all six. Happy travels.