In order to get the most out of the XL650 you need the case feed. The press does not come with dies so if you don’t have any, they will be a must. Dillon makes great dies. Nothing else is a must have, the press works well as is.
A spare parts kit is nice to have, Dillon will send you parts free, but you will have to wait a couple of days to get them. And that can mess up a reloading session.
You will eventually want conversions for all the calibers you will be loading. It is nice to have tool heads and powder measures also. You don’t need to get them all at once unless you are rolling in $$$.
There may by other things you want but you need to decide for yourself. For example some say the roller handle is a must, but I have never used one, I just put a sock over the stock handle and it works for me.
It is an awesome press and I have been happy with it since I bought one in 1994.