I have changed both mandrels on my Craftsman. One broke when I ran over something and it bent the blade and messed up the bearing in the mandrel. The other just failed. Both were the type that did not need to be greased as they were sealed.
I would recommend purchasing the complete mandrel, remove the old one. I would also get the type you can add grease to periodically. I am not a lawn mower mechanic but try to do my own work when it is simple.
You need to remove the deck...makes it easier. Be careful when you order from Sears, they screwed up my order twice. I brought the old mandrel to a lawn mower shop and they fixed me up.
I would figure it took me about an hour, including getting the deck off an back on. Mandrels were about $50 each. One of the sealed ones I got from Sears, paid $50 for went bad after about 8 hours mowing, that is when I went to ones you can grease.
The reason I tell you to bring your old one with you is because some have 3 bolts and some have 4.
Just my 2 cents worth...