The only negative was that one of them had an issue with the motion detector malfunctioning after a few months.

.-  Given that were are likely near the top of the market (just statistically speaking), I'm not a fan of projects that will take a long time. 

Some even chase after them, ha.Lots of places to invest that have never been hit by a tornado and statistically low to be hit by one.

So is a 30 year and just wait for rents to rise over the years to give me the cash flow I want for retirement a good idea...or is a fast pay down and cover the negative for a substantial rental income in the future a better idea?

Equity creates negative cash flow on a property.

You may see some effect due to age of credit history but probably not enough to take your score down to where you are seeing negative impact in your interest rate.As @Anton Ivanov correctly stated you will want to have a game plan for how to handle your DTI.

Vacancy at 5% of gross rent would equal $3,285 / yr, which ends at -$-473 cash flow.So if PM and Vacancy were incorporated at current rent rates, would lead to negative cash flow.What is RUBS?

Start pushing your limits too hard, go outside of the box too far, or abandon your core competency...this is the start of trouble.Having survived at least a couple of cycles I can say that chances are you'll feel pain when it happens but if you were disciplined when you bought your properties and stuck to what you are good at you can survive.In 2005 I regularly attended REIA meetings where people were bragging about the rental they just bought with $500/mo negative cash flow despite a neg am loan and nothing down. 

I have a seller that is current. He is upside down 30k on his home and wants out. Piti is 500,  market rent $1000. If I took it sub2, and rented it, would I be risking too much on the due on sale clause? If the bank d...

Goes back to how inaccurate are those statistics when people can't correctly identify a dog.I like Shaun's comment about requiring paperwork showing the dog has done obedience training and/or successfully earned the Canine Good Citizen (CGC) title from AKC (which isn't difficult at all, even the advanced CGC title).