Which will be best for you will depend on your specific situation and goals.The advantage of using the retirement plan to invest in real estate is that you will have access to the full amount of your retirement capital and any growth through cash flow and appreciation will be tax deferred.  

Purchase prices are considerably lower than Los Angeles and other cities in SoCal but the rents are still high enough to produce high cap rates compared to the rest of the state.

We would like to target homes anywhere from there down the 95 corridor to the lower part of the North Shore Beverly --Peabody, Danvers, and Salem areas.  

I'm not going to try to sugar coat it:Is there upside in rent as that's one of the most important things I look for.Based on my experience, the cap ex is probably high.You won't be able to get a 40 year loan so that pushes your mortgage amount up and your cash flow down.You should be able to get lower than 5% unless you are only going into a commercial loan.Terrible cash on cash returnany property I buy needs more than $5,000 in repairs. 

A 50K duplex will most certainly be in a lower income area.  

I am a broker so I have access to MLS and have a jump on good deals. 

You don't have to go 80/20, you can go 60/40 for a lower payment and (sometimes) a slightly lower rate.

The neighbors get foreclosed on more than typical neighborhoods and that keeps values lower for longer.2) HOA association decides to levy a $20K assessment on all owners in the neighborhood in order to upgrade everyone's windows, decks, roofs, and siding.3) HOA increases monthly dues by $250 per month in lieu of a large special assessment. 

It sounds like the lower response rate of postcards vs the yellow letters is a good thing?  

It's higher in rougher areas and lower in nicer areas.