I would not recommend an Salesforce to SAP integration from scratch. This is too much effort and you need to take care of things like error handling, delta load and security. Instead I would propose to use a tool that was made for such scenarios.
In case you only need data replication, you can go with something like SAP CPI or Mulesoft. They even offer templates, that make your life easier. You should be aware that these options can be expensive in regards to their licenses/subscriptions.
Another way would be a solution like Vigience Overcast, that offers data replication as well, but also real time access to SAP from within Salesforce. Overcast goes beyond exchanging data between the two systems, but also allows you to create the user interface. They even have pre-defined, ready-to-use apps for the most common use cases.