Hi Steve
The biggest issue with connecting after going live is ID mapping. For example, with materials. Let's assume you have 1,000 materials in C4C already, and 50,000 in ERP. For the 1,000 in C4C, you need to know what material it maps to in ERP. How did the materials get there, were they loaded or created? You'd need to consider the data as well as the ID mapping. The accounts/contacts will also have similar considerations. Once you integrate, will accounts and contacts still be created in the cloud? If so, will they need to be integrated? I know when a contact is created, if we know there is integration we grab the number range that will be assigned to ERP - so the number ranges for accounts and contacts that need to go back you will need to be careful. For the ones created, as long as ID mapping is there, there won't be a need to create a number range so it will be OK.
I would start with an object that only replicates one way - like products from OP to C4C (for example, ERP to C4C).
-ginger