One option is you could import the leads into Salesforce.
And then have the sales team gradually work through the leads by emailing them for a meeting as well as linking to some sort of landing page with an offer.
Not the fastest method, but you won't have to worry as much about marketo deliverability and spam complaints etc.
Check out my blog post on tracking non-Marketo sends: http://blog.teknkl.com/tracking-clicks-non-marketo-emails/
(And I do agree that you should not use Marketo to send unsolicited emails to a rented list, if you send at all. CAN-SPAM is the weakest of your worries; EU and CA law is mich stronger.)
And with the upcoming GDPR in the EU, you can simply and completely forget about doing this in Europe.