How does the current rent of $500 per month on this unit compare with others of the same size and condition in the area?

The credit union is pretty small, so they're hustling harder than the larger and mid-size banks to get loans, so they are willing to be a bit more creative.

Without system size and other info it is hard to say if that is reasonable or not

Whether your exchange is $100,000 or $1 million, DST investments can be sized to fit your exchange.

What kind of lots, and what size?

Screening and sizing people up comes with experience.

Was just offered 340k cash for a property in SW Miami Dade, although a substantial amount and much more than what the property was paid for we would be giving up the opportunity to double or triple the size of our portfolio.

Total lot size is 1.5 AC.  

As I recall, they told me that an average size house would cost 5% of the sale price.  

They know what size house is the best rental in their area.