Pokemon Essentials received a C&D by Nintendo so we do not support that anymore. Author is also old so its expected for links to disappear.
Regarding download links, your best bet in most scenarios is posting if anyone still has a copy that they can re-upload and/or messaging the original author (though this will really only apply to us more active users). Otherwise, Google search. No results? Script is lost forever.
Quick google search for
RPG Maker XP SDK and this was at the top of the list:
https://save-point.org/thread-2361.htmlIt's a 10k lines script that makes the default scripts more modular (intended to be easier to make script add-ons). But this caused a rift in the dev community as some scripts just do not work with SDK. We are a community that heavily despises SDK and you will not get support for it here. Your mileage will vary.