Get the Genuine Mercedes ball joints from your favorite online dealer, like
MB of Laredo, or
MB of Covington. Since you are in SoCal, maybe just go to the MB Classic Center as detailed
in this post. Ball joints are still available new. If you plan to DIY this job, some special tools are needed.
Coolant overflow - you need to be more specific. What exact part are you looking for? If you want the plastic reservoir with the radiator cap, that has been NLA for a while. Don't replace yours if it isn't leaking or otherwise damaged.
Rear SLS hydrolegs have been NLA for several years. There are no replacements available, and NO shop that properly repairs them. What is the reason you want to replace them? Need more details here also.
For ALL of the above, and other questions, please search the forum first. Almost everything you want to know about the care & feeding of your 500E has been discussed repeatedly over the past 2 decades.
Welcome to the forum!