I have the number one rule, Don't mix Money and Family together. 

Face to face, call, letter?

Do you recommend a generic mail out or personal hand written letter.

Next time they have to pay to the letter of the lease and mine are very punitive. 3rd time directly to the eviction notice. 

I am presently testing this recipe promoted on YouTube and other home repair site:-Mix equal parts sugar and boric acid powder-Place in jars or small containers near infected areasI placed my test mixture into a small petri dish and so far, haven't seen them drawn to it like some have claimed. 

If they like the deal, they will potentially submit a letter of interest on behalf of you.

I have tried sending mailers, hand written letters and left sticky notes on their doors stating I am interested in purchasing their house with no luck or call backs.

(I will use it sporadically the rest of the time).I have asked my agent to get a letter from HOA in reference to the timing of the increase.I am “reportedly” buying it from an elderly couple ,the house inspection was fine but still does not feel right..

So to your last question...again it will depend on your DTI.If you are planning to add a flip to the mix, just run with a hard-money lender; they are not going to be too concerned with your DTI in that case; the asset backs the loan.The best thing you can do if you have a refi or two coming up is to keep in constant contact with your lender and ask them if you are going to be in a tight spot before pulling the trigger on the next deal. 

Only challenge was that I had planned a trip prior to closing and that made it difficult to get everything prepared for the transferring utilities, preparing intro letters, etc.