Sounds like that can easily turn into a never-ending business developing and/or business intelligence project for the client you've mentioned. I assume this is not what you want to achieve, otherwise you would be a cofounder at CRM consultancy startup.

- Be as specific as you can about the scope of data you will communicate between and what exactly will be a part of this integration (try not to get involved with anything other than that). - Expect a lot of traffic as CRM's tend to be loaded with useful and useless data which just keeps growing. - you can expect that your client will often want to increase the scope of data fetched or sent to CRM. - you can expect that your client will often want to trash or change data fetched or sent to CRM.

All CRM's have out-of-the-box API which you will probably have to use, so look into that. Usually you can CRUD a record in CRM by providing ID and type values of that record.

Good luck!