Microbial influenced corrosion (in addition to others) often manifests with creating blisters on the outside of a pipe as the interior of the pipe develops a depression.

That might influence how you would build a portfolio.

And if it is not, how does it influence me?

That's just my influences speaking.

Further more, some ideas to generate leads would be finding tired landlords, Send mailers to out of state land lords, Drive neighborhoods looking for vacant and abandoned homes, Talk to the mail carrier and other delivery people because they would know the empty houses in the area, Mailers to Landlords who own more than one property with back taxes, Mailers to Landlords who own tax delinquent multi-family properties, Place “We Buy houses” flyers/business cards at unemployment offices….a lot of people there could be having trouble with their homes and may need a solution, One-page Flyers posted on bulletin boards at large stores-get approval from manager though, Using Craigslist to find sellers, Send emails to craigslist ads with apartments for rent (the emails go to landlords who may be looking to sell, or buying more properties) Target people who have been transferred (job) and must sell NOW, Military transferees, Move-up buyers-People that need to sell and get a bigger house but feel stuck free & clear houses because they might sell with seller financing, Network with moving companies, Network with termite (WDO) and pest control companies, Network with real estate servicers – title companies, insurance agents, mortgage brokers and lenders – everybody knows someone in distress, Network with credit repair companies to assist their customers that must sell, Funeral homes and obituaries, Houses with boarded up doors and windows, Houses with blue tarps on the roof, Network at your local real estate investment or landlord association, Network with your sphere of influence - hand out your business cards to everyone you know- you may learn your neighbor has a friend that needs to sell a house etc.

You are at mercy of the HOA, unless you can find a way to control/influence the HOA.  

Typically you want to have the tenant pay things that they directly influence through the amount of use: water, electricity, gas.  

Note that I said cap rate range because it's not an exact science and there are a number of factors that can influence this. 

Mortgage rates are ultimately heavily influenced by the Federal Reserve but the mortgage rate itself is ultimately determined by the secondary market buying and selling mortgages.

But I’ve also been influenced lately by podcast and local meetup buzz about moving from short term to mid term rentals.