How many miles on this potential purchase, and what do you know about it's history?
DuRon's comment about sludge is something to look for. Thing is, you'd have to pull a valve cover to get a good look at how clean the engine is, or isn't internally. The 4.7 is a good engine, but it's not a 5.7.
Do you plan to tow at all? Have you run the VIN number yet to see what options it left the Newark, DE plant with?
Make sure you check the engine from a cold start. Make sure you don't have any exhaust leaks at the manifolds.
When I bought my 2004 SLT with the 4.7 and 190k miles it had broken bolts. Sounded nasty for about 5 minutes until it warmed up and the exhaust manifolds sealed up. It ran pretty good for the high mileage.
I don't recall any 4.7s having such issues with regularity. I believe there have been a few of them that were way up in the miles, that experienced a lifter or valve issue. Nothing like the 5.7 and it's valve seat/valve spring/cam and lifter debacle though.