Hi Judi,
Phoenix changed from the E350 to the E450 on model 2551 around 2007 because the E350 was getting close to the maximum allowable load. I cannot say that it is a bad idea to buy an older 2551 on the E350 chassis. It is simply less desirable. If I were you, I would call the factory and ask to talk with Kermit. He made the decision to switch from the E350 to E450 on that particular model. He would have the best advise for you.
The general consensus is that the E450 is preferred, but an E350 is not all bad news because it will yield about a 10% increase in fuel economy along with a softer ride.
Buying the rig used, there is one primary thing you will want to know about. That is whether an aftermarket heavy duty rear stabilizer bar was installed. If not, I would not consider it a deal breaker, but rather negotiate $400 less to get that component added after you purchase the rig. Actually I would advise to add more than that rear stabilizer bar to further improve handling. I'd replace the front stabilizer bar with a heavy duty one along with better shock absorbers and steering stabilizer. Hopefully the rig you are considering has much or all of that already done for you.
