I am today years old in terms of this forum and it's already provided a wealth of super useful information.

The options contracts I've done in NC (only 4 or 5, me as seller, all post-GR) set the 'contract price' and terms.

Long term friendships with local realtors How did you finance this deal?

I suspect it will not be popular long.

Am thinking of converting a long term rental to a medium rental. 

As I get into short-term rentals, I’m curious: What should I look for when picking a location?

It would help us to understand the attainability of your goal if you would mention how many flips you have under your belt already.Assuming you are experienced, then I'd say move forward, full speed ahead--you can do it.If you are inexperienced, I would respectfully suggest you change from a $$$-oriented goal to a ### of transactions goal.Why?

Everything here is a trade off and if you're interested in long term planning, it's best to ensure you fully understand how it all works before jumping in.

At this time, the types of properties that I'm interested in pursuing are primarily multi-unit rentals and short term AirBnB properties.

Places like Maplewood, Richmond Heights, Affton, etc. would be where I would start (if looking at long term rentals).