To implement and test my application for Xprotect Express (plugin or component) must I install XProtect Corporate? Need to buy a Corporate license?
No, if developing for a specific project where XProtect Express is used it is best to develop towards and test with XProtect Express.
Now if you at one point wants to sell your integration to a broader audience using the whole range of XProtect versions, we would recommend having a XProtect Corporate test server. See this knowledge base article on the subject: - https://force.milestonesys.com/support/articles/en_US/FAQ/About-MIP-SDK-compatibility-with-XProtect-product-versions
At that time you might also consider becoming a Milestone Solution Partner. - https://www.milestonesys.com/community/become-a-partner/solution-partner/