If you want to set up lead assignment rules in Marketo based on 700 county values and 150 salespersons, yes you can do it.
Assign 100% in Marketo
Campaign will look like this:
Smart list: (trigger to assign into SFDC, often "New lead is created" or "Form is filled out") (filter for leads with county value if you don't have a default owner)
Flow: (150 choice steps, each says "if county is (x,y,z) then assign to (a))
Assign within Salesforce:
Although you said you don't want to assign within Salesforce, its worth noting this is a bit easier if you assign the counties/territories in Marketo and then create lead assignment rules in Salesforce based on the county values.
Salesforce is also a bit messy. The assignment rules would be manual.
Wrt the Marketo solution, i'm fine if this was to be a 1 time activity.
The problem is maintenance. How do i ensure changes in the territory structure is reflected in the flow action?
Ideally i need this to be in an excel sheet & have Marketo look it up and dynamically make the assignment.
Tried a lot of stunts, namely trying to assign a lead owner via a token, but that also didn't work