When enabling Multi-Org in Marketing Cloud Connect, it's important to consider:
The Multi-Org configuration does not support using a standard profile and preference center across connected business units. Each business unit operates independently.
Admins must consider alternative strategies for managing subscriber preferences across multiple Salesforce orgs linked to different business units in Marketing Cloud.
Reference: Salesforce Marketing Cloud Connect Setup Guide

As the Marketing Cloud admin plans to expand the data model to include SMS and Push channels, it's important to prepare the data model for high data quality:
A). Remove nonessential data for marketing purposes: This helps streamline data management and ensures the data model is not cluttered with irrelevant information, making it easier to manage and use.
B). Identify and assign appropriate keys to tie records together: Proper key management ensures that different data sources can be linked effectively, facilitating a unified view across different marketing channels.
C). Normalize data and fields to prevent redundancy: Normalization reduces data redundancy and inconsistency, which is essential for maintaining data integrity and efficiency in a multi-channel environment.
Reference: Salesforce Help - Data Management Best Practices

To enable Sales Cloud users to create and send Marketing Cloud emails:
User Mapping: Ensure there is a one-to-one relationship between Sales Cloud and Marketing Cloud users to facilitate seamless functionality and data consistency.
Deep Linking: Enable deep linking in Marketing Cloud Connect. This allows Sales Cloud users to access Marketing Cloud features directly from within the Sales Cloud interface, enhancing user experience and efficiency.
Reference: Salesforce Marketing Cloud Connect

Journey Builder and Mobile Push are two products that would allow them to send push notifications to customers with their mobile app. Journey Builder is a product that allows marketers to create and execute cross-channel customer journeys based on events, activities, and goals. Journey Builder can be used to send push notifications to mobile app users as part of a journey. Mobile Push is a product that allows marketers to create, manage, and deliver push notifications to mobile app users. Mobile Push can be used to send push notifications as standalone messages or as part of a journey