I created a data dictionary in Excel with tabs for the field in the Lead, Account, and Contact objects. I then ensured that the fields I wanted to the same Marketo lead field (e.g. Annual Revenue) had the same name in each SFDC object. I also noted which fields should be Visible and Read Only to Marketo. Finally, I noted which input sources (into Marketo) should be blocked for each field. Not sure what you mean by "map priorities".
Thank you, Elliott. I've started something similar in Excel; it's good to know I'm on the right track. When I say "priorities" I mean Rules. We have some overlap in multiple data solutions and sometimes the duplicate data doesn't match. I want to set up some if-then statements to avoid losing data accuracy. (e. g. if fields don't match, trust List A data and ignore List B.)
My company offers a free solution for loading data which also makes it pretty easy to map data fields. You'll be able to import all kinds of records (lead, contact, account, etc) into Salesforce from an excel csv file. I just updated over 120k records and it worked like a charm. It's accessible via http://www.dataloader.io
Good luck with your import!
Watched the video - That's Dreamy! I wish we were using Salesforce, but that's not part of our current project scope. Thanks anyway!