In reading as much as I can about marketing I have determined that I might be better off concentrating on specific zip codes with more letters per property.

I don't think it's good to rule the first three out completely, but if you are trying to further your dollar, and your time, concentrate on those individuals you feel are motivated and not making the effort to call you.

In that case which 'groups' should I concentrate on?

I would concentrate in the area of high equity properties and also htose with major repairs needed as most investors like the ARV model.

That is about 1 dollar apiece.Should be 50 cents if you do yourself.If you have limited funds don't start out at 200.Do a more concentrated and highly targeted approach.It's better to send 100 letters to quality prospects than 1,000 letters to the wrong audience.

All of my personal SFR portfolio is concentrated in South Forsyth County.

If that were true, I would only invest in apartment buildings that had a high concentration of young woman that . . .

I'd like to do assignments/same-day closings/ flip deals in both countries, but I would like to concentrate more in the US. 

I've saved enough money in my bank to put down a good amount for a property(ideally to start out with a wholesale), but Im also considering doing an MBA with a concentration in real estate.

What areas are you concentrating now days?