Get estoppel letters from them verifying their rent amounts, the remaining terms on their leases (assuming they even have leases) and any security deposits being held. 

Tony will be listed as the contact on the appraisal engagement letter the lender sends out.I also recommend putting together a similar professional packet for your lender including the analysis, rehab costs, tax returns, personal financial statement, real estate portfolio and bio on your self.  

We need to send a cover letter with several pages on the bottom as an appendix.

Get any financing done and get a pre-approval loan letter from a lender that states how long that closing will take and the maximum amount you can borrow.  

Do I send a letter or knock on doors to inform tenants about how/who/where to pay rent? 

Send them a letter (not yellow letter because they are getting dozens of them) telling them you found their house to be apparently vacant.

I'm new to real estate investing and would like to know if before going into a contract or start looking for rehab deals do I need to first get prequalified by a hardmoney lender and get that letter to provide to agents or they want a deal under contract first?

But  to get to your question, just send them a letter stating you are not renewing the lease when it ends.

Looking for advice on which address I should send a "I want to buy your house" letter to. 

I have sent letters, but phone numbers are not available for all but a couple.