Depending on the class of tenant, you may need to up your vacancy percentage and you may have some "bad debt" (rent that is never paid). 

When performing a quick preliminary screening on those websites (i.e. to exclude obvious bad deals), do you usually use the 50% rule (or similar % rule) to estimate expenses?

Second as a contractor  I have found lower cost subs equals lower quality , or uninsured , etc. 

Upon closing and finalizing the loan, I realized the mortgage / taxes / insurance / PMI totaled out to $640 a month, which isn't bad in my opinion, but a little more than the $550 I was hoping for.

It's not that hard, but if you've never done it and are nervous, it's not a bad idea to hire a realtor to walk you through it.2) You pay a title company to research the title and get title insurance, same as if you used a realtor.3) Not really - but like you said, guidance and knowledge are worth a lot.

Personally I think it's tough to say a "good or bad" investment based on the zipcode alone.

My concern with multi unit properties is the quality of tenant, that and cash flow on any properties in our area is very hard to come by.

Tools/Equipment/Contents7.

A referral fee is a not a bad way to go since you have a lot lower risk and it sounds like you might need to learn more before taking on a project of this size, no offense intended.4.

People who can afford deliberately stop paying mortgage to get out of bad investment.