Important points
The most important points to consider when integrating from with Engage from scratch are:
Your integration must be synchronous, meaning you'll need to wait for Engage to respond before proceeding in your flow.
Any contact IDs returned from Engage should be saved in your local systems and accessed in the future to avoid unneeded traffic to Engage, to simplify and speed up look-ups, and to generally improve your customer's experience.
Have a retry policy in place, based on the recommendations given here.
Implement a local backup of changes for when the connection to Engage is down for whatever reason. Don't make your vital business processes depend on Engage always being reachable.