Generally speaking, a lead is an unqualified opportunity to make a sale. Once you've established contact and you have an idea of what you will be selling them and how much the deal is worth, the lead progresses to an opportunity. Ultimately though, it is up to you where you define the transition.
In your example, you could import the database as leads, customising the lead form to ensure all information can be brought in (you could also use mapping to ensure the information transitions to the account/contant/opportunity records when converting).
You sent up a campaign and mailing list for your leads which you've filtered out and you set up phone calls and send the emails. For those not interested you disqualify the leads.
For those interested, you convert the leads to account/contact/opportunty records.
When you want to resend the emails, you have the campaign and mailing list to work with. Filtering out the disqualifed leads from the mailing list should not be a problem with the filtering tools.
Checking a disqualified lead was not later entered manually would be troublesome but I'm not sure I understand why this would occur.
Checking for how often we send offers would be a case of looking at the entity activity history or looking at the campaigns they have been part of.
Leon Tribe