Best way to deal with prospects

We have sourced prospects from one of our databases (and will continue to do so) and are looking at making sure they are quality prospects before we sync them to Salesforce as to not riddle Salesforce with unclean data.

 

Is there a way that you can 'stage' in Marketo and not sync with Salesforce until we are sure that these are engaged with us?

 

We are planning to send an email campaign and use "Opens email" as a trigger to then sync to Salesforce.

 

Currently I am looking at using "Change Data Value" but am unsure of the best attribute to be using for this? 

 

I have already uploaded the static list to Marketo of the first lot of prospects under the assumption that these would not sync automatically to Salesforce, however it looks at though have.

Unsure how I can make sure that these don't sync for the future prospects we upload? Is there a way I should be blocking these? 

Any suggestions would be appreciated! 

Thanks!