There are way better people to talk to that are based out of here and know the market like the back of their hand. 

I am in contract on a duplex in Grandview where I plan to live in one side and rent out the other.  

If I were to approach a seller of a $200k home, and they had a remaining mortgage of $100k, could I get a new bank financed loan for that $100k, and have the seller finance the remaining $100k of equity to me?   

Lots of generalizations in there.

It's probably best to have them pay out of pocket for a smaller cost like this.

I recommend starting with some BP podcasts and starting to figure out what your skills, interest and capital align with.

the  I have seen a lot of properties come and go.  

Wait, I get it, their online reputation will be enough of a guarantee for you!

So, you can do a land contract for 12 months, which should allow lender to credit you full value of the land vs the purchase price, but the loan will pay off the land contract, so the lender maintains first lien position.Can't help you with the rest of your question, but you should be able to search for "CPA" here on BP to find experts to reach out to.

I think you can't go wrong with a long term rental, but don't count yourself out of STRs because of your location.