As I'm sure you know, inventory is low and prices are high in Seattle and my company has access to a fair amount of pre-market listings!

I suggest limiting your search to a closer range (if you plan on living in one unit) as I suspect cheap gas prices are not going to be around forever. 

A properties have the lowest cap rates, ranging from 4 to 6.

Palm Beach County is a big spectrum of areas, ranging from the low low to the super high.

Im a local Realtor and can tell you right off hand that you're not going to find properties in that price range in the areas you mentioned.

What do you feel are some of the upcoming suburbs with good cash flowing properties in the 100 - 150 K range?

It is a great feeling to roll another hard-working property into the inventory! 

If you are not within that range I would say you should hire a management company for the fact it'll take you longer to get to the issue if you needed to be on site of the property with a maintenance issue.

I have checked in the new jersey area and multi family inventory is scarce in the desirable neighborhoods.

I'm leaning towards the $100-150 thousand (ARV) range for the properties I target.