We have the baby brother - 2003 Phaeton with the 300 Cummins ISB. As second owners we bought with out ACCURATE records. Best advice is to not believe everything - cjeck and double check. All new fluids and filters may have been what he paid for (with stickers to document this) but actually most was never done. So be prepared to do some serious upkeep to be certain.
2006 we replace an alternator that failed with a more robust and better 'paired' one. Tires of course need to be closely examined - we have now replaced all six. The only serious defect was a slide topper that delaminated and was a known defect that we missed the warranty on. Beware of advice to owners of newer engines and trannies that MAY not apply to an 03. FL has told us to test SCA of antifreeze for example, but our 03 is not in that category. It likely still has dexronIII and that is no longer easy to acquire so transsyd is a good change. Read the forums, learn as much as you can, and enjoy a good coach.