Not high on my priority list. do you have dog or dogs.. 

Is there any way to use the equity in my current property to put towards the new property, or am i basically stuck selling the duplex and rolling it into the larger 6 unit rental?

I don't let the tail wag the dog when it comes to investing.

The larger ones are selling for 300 to 350 per square foot new construction depending on the lot.

Kansas city is a great market to invest in - are you thinking smaller (1-4 units) or larger (5+ units) multifamily properties?

And south of that street is going to be newer, larger homes, a tier up. 76542 is actually very similar to that general rule as well.  

What are some pros that I can highlight from the seller’s perspective.Here are a few that I’ve come up with:1) monthly income at a higher interest rate;2) avoiding a larger tax bill by reducing the purchase price (on paper);3) a promissory note that they could potentially sell down the road.Any other major ones I am missing?

There are lots of larger investor meetups around Charlotte I'd be happy to share!

For doing an STR well the larger properties are best and it needs to stand out amongst competition to perform well. 

You need larger down payment: 15 to 25 percent down, depending on your profile and property type.