Since their lease is ending soon, you may want to start with a demand letter for the owed rent.

Lenders will want to see 2 active open trade-lines for the past 24 months with on time monthly payments or a credit reference letter from a creditor. 

Even if you send out one letter a day, that is five a week, twenty per month. 

Having a Pre-approval letter or better yet a loan commitment and a Banker to call and reassure the sellers agent you can close in 30 days or less also offers a huge advantage.In this market using a DPA is not a bad idea since rates are high and when they drop you simply refinance for a lower rate and payment. 

We spoke to each tenant and gave them a letter explaining our potential plans. 

You may get a couple more letters, one from the old servicer and one from the new servicer, if that has changed.You are the trustor, so nothing changes for you.

Direct mail has a very low response rate and I mostly just see the big boys who are going 7 figures do it, who don't mind spending 10-20k+ month on blowing out towns of letters/postcards.

Our rental is paid for and has rental income of $2200 month and our primary has a mortgage of $1800 so we are currently living for free basically.we have a credit line on our rental for $275k and recently got a letter from our mortgage company offering a cash out refi on our primary of approx $300k and still be able to keep our current interest rate on the remaining balance of the mortgage which is mid 3 %.We are trying to grow our rentals and struggling to figure out the best way to do that.

Two dates after closing a letter arrived addressed to the previous owner from a lender about late payment on loan.what steps to take?

If they use your credit and income docs to formulate an offer, you can ask for a pre-approval letter (if you want), or just analyze the pricing and ask for a pre-approval from the top 2 lenders.