$7000 for a used 4.8is is dirt cheap! A dealer long block cost $19,000 at the dealer. They are very rare engines and hard to find. Surprised you even found one.

FYI, I am an engine builder and tossing in new rod bearings will only be a temp repair. Usually a week or two from most my experience. But there are always exceptions to the rule.

Are you sure it's not a cat converter rattling? It's pretty common. Sounds like a rattling knock in midrange rpm.