The best advice I can give you at this stage is to ensure your data is as clean and accurate as possible. You never want to start with dirty data.
Also, in our environment (Marketo/MSD CRM), leads aren't synced to CRM until they are actually marketing-qualified. That being said, it will be important that you have clearly defined processes/SLAs in place between marketing and sales. For example, we found some sales execs entering their contact data as leads - all they had was the name and company (and entered "N/A" for the email addresses). Since this all synced back to Marketo, it caused many issues. So make sure that alignment exists between sales and marketing.