I've had exhaust manifold problems on 3 of my trucks, all are work truck towing heavy loads... we keep our trucks for 500K miles and work them... .
You don't say the mileage but I assume from the manifold its been a work truck... I wouldn't buy a stock manifold, they just tend to warp, snap off bolts and crack... . get an ATS or BD 2 or 3 piece manifold and be done with the problem... .
In my signature you don't see the 01 we ran for 450K miles towing a 20K lb trailer, we sold that truck and also you don't see 2 others we own... just not enough room...
On a whole these are great trucks, must better than the Fords I traded off to buy these... we really like the 08 5500, it gets better fuel economy than the F550 we replaced (500Kmiles) and it now has 110K miles and seems to be running at least 70% less to run than the Ford was...
If you think its a reasonable truck, and runs well, has some track record for maintenance I'd consider it...