I've also been told not to buy a used rotary unless you really know what to look for. Friend has been burned replacing cam tracks and followers on two used rakes now (His were claas).
All the models I've been told to be very careful of have exposed cam tracks.
Yes make sure the center section is still sealed. Kuhn has little plastic spacers between the arms. A friend of mine loaned his out years ago and the people bent an arm or 2. When they replaced it, we didn't notice they left the spacers/fillers out where they had to pull the bent arms out. This allowed dirt/dust in and ate away the cast track in 1 section. I just rebuilt the rake for him this winter, put $800.00 in parts in, we could have done it for less, but as long as it was apart we replaced everything.
The center on the Kuhn comes apart hard, no good way to pull it apart. Once apart the job was easy.