I think if you buy them as an investment you are likely to be disppointed unless they are in exceptional condition and you manage to steal them.
I own two COE's, a 6400, a 4400 and both a GMC 350 and a 450. Big trucks are great and six may not be enough, but I would never buy one thinking I'd make money on it. Either they are a personal passion and worth owning on that basis or likely a waste of money.
If what you care about is saving them, then buy them, knowing you may loose a little money while finding them a home with someone who wants them.
