Legally you need to check with your companies policies for turning off servers. That would answer most of these questions.
If you are returning these servers to IBM off a lease or going to sell them to a third party chop house you need to get your data off. The best way to do this is keep the load source disk from the system and either destroy it or just keep it on hand.
I f you are going to part out the systems then part them out by product type. Let the system go but pull out the disks and other items. Make sure they give you a great price.
Also if you are returning it to IBM from a lease you will just have to let IBM pack it up. If you want to protect your data then scramble the disks all over and clean the system with a scratch load of the OS.
Other than that it’s all pretty easy unless you have some more specific questions.