What else do you need?
Avatar requirements
This part is outdated and currently being rewritten due to the introduction of VCC.
For more information, please visit https://vcc.docs.vrchat.com/
When looking in the readme file and the description on booth.pm store page, you will be looking for things the avatar project requires, items like these shaders lilToonShader, PoiyomiToonShader, UnityChanToonShader, ArkToonShader, JewelryShader, and scripts like Dynamic Bones and SDK version.
VCC Creator Companion
Unity version
The current Unity build used in VRChat can change over time, at the time of writing the version is 2022.3.22f1.
But if you want to be absolutely sure about the latest version in use then please check the creators.vrchat.com website.
Some older avatars on booth might still use version 2019.4.31f1, VCC allows you to create a project based on this version. (it does require you to install this version of Unity as well) But it is recommended to upgrade your project to the latest version. Information about upgrading to 2022 can be found here.
SDK version
SDK is usually very straightforward, if it only says SDK then it usually means SDK2 and otherwise they will specifically mention SDK3. Please be aware that SDK2 is considered obsolete and will no longer be maintained for features. SDK files can be downloaded for free from the VRChat website and are listed under "Manual SDK Installation".
Keep in mind that SDK2 is now in the status of being deprecated. You can read all about that here. If you are new to uploading VRChat avatars then I would highly avoid acquiring avatars that list SDK2 as a requirement.
Shaders
Shaders can usually be found on booth.pm or GitHub. If you can’t find the shader because the website/project no longer exists then always double check if the shader is included with the package. Google can also be a great help to find a specific shader.
Dynamic bones
Dynamic bones is a separate package from the unity store website. You can buy this for +/- 20 dollars. If you are stuck on a budget then it's worth waiting and keeping an eye on it as it often goes on sale with a neat 50% discount.
Once you own dynamic bones, you can apply it to any unity project you want. Including your own projects when you perhaps make your own avatar in the future. There are also plenty of guides on YouTube that will help you add dynamic bones to avatars that don’t use Dynamic Bones.
NOTICE!!!
Please be aware that Physic Bones has been released, this feature is provided by VRChat for free and is included with the current version of SDK3. This replaces Dynamic Bones!
The sole reason you would still need Dynamic Bones is when you want to import an old avatar with Dynamic Bones and convert them to Physic Bones. This conversion can't be done without the Dynamic Bones package.