Here are a few considerations to keep in mind while designing your integration solution for Dynamics AX:
Simplify the overall architecture and try to reduce the number of integrations between applications wherever possible. It is one of the areas that cause recurring issues in production.
Define clearly the master system for each data element, even though you may have it stored at multiple places. In some cases, this may have to be defined at the field level. For example, a customer master is stored in CRM and Dynamics AX as well. CRM might be the master for all the customer information except credit limit.
Ideally, you should avoid duplication of data across multiple systems, although in some cases you cannot avoid it for business reasons or for systems to work. For example, customer records are required in both, the CRM system and Dynamics AX. However, you can opt not to integrate the CRM-centric customer information that may not be needed in Dynamics AX...