As of the Winter '24 release, Salesforce introduced the ability to send outbound messages directly from Flow. This makes Flow the best choice for automating the notification to an external system when a lift is sold, fulfilling the need for both checking the custom To Be Installed checkbox and sending the notification the next day.
Flow: With this new functionality, Flow is now the most versatile automation tool for this use case. You can:
Create a scheduled path that triggers the next day after the lift is sold (e.g., when a record meets the criteria such as the checkbox being checked).
Send the outbound message to notify the external system about the lift installation.
Perform all of this declaratively, replacing the need for Workflow, which previously was the go-to for outbound messages.
Why not Workflow anymore? Although Workflow still supports outbound messages, Flow is the preferred tool now, as Salesforce is deprecating Workflow rules and Process Builder in favor of Flow. Flow also allows for more complex logic and actions, providing flexibility in future-proofing automation.
Winter '24 Flow Outbound Message

To manually share individual records with specific people or group of people on a case by case basis, users can use public groups, roles, or users. Public groups are collections of users, roles, or other groups that can be used to share access to records. Roles define the level of access that users have to their organization's data based on their position in the hierarchy. Users are individual people who log in to Salesforce and have access to records based on their profile, role, and sharing settings.

Creating both customer and dealer as accounts, creating account teams on each account and associating the dealer records with the parent account would allow UC to track each customer and dealer separately, as well as their relationships and reps. Account hierarchy is not suitable for this scenario, as it is used to show the relationships among parent and child accounts within a single company

Workflow rule and an outbound message or Process Builder and Apex Code are two ways to integrate Salesforce with an external web service. Workflow rule can define criteria and actions for triggering an outbound message, which is an XML message sent to a designated endpoint URL. Process Builder can invoke Apex Code, which is a programming language that can make callouts to external web services using HTTP requests. Flow Builder and an outbound message or Process Builder and an outbound message are not possible combinations.

A managed package from AppExchange is a custom solution that is developed by a third-party provider and can be installed in Salesforce. A managed package can be easily updated by the provider without affecting the existing functionality or customizations. This makes it suitable for the app builder's requirement of ease of updates. Option B, C, and D are not as easy to update as a managed package.