The sharks at the meetups will be obvious avoid them and build relationship with everyone else.

Unless you also had plans for new builds in the near future, in my opinion, there's much better investment options available in the immediate area

My question is while I was looking on redfin, I found a big 14-unit buildings that is for sale around $3 Million.

The main thing you will need to do before renovating any house or building is to make sure you have a good team in place (an administrator or a licensed care home manager).

What are the pros and cons of building from scratch for the purpose of STR?

You don't want to rush to partner just to get your next property.If I was new again now and didn't have the capital to get my second one, I would go to every meetup in my area, meet other investors, and build relationships.

Build the relationship with them.

They don't buy a lot, and even when they or any affordable housing developer buy land, it often can take forever for them to consider the property and then the actual escrow can be long if they are building the capital stack to buy the property. 

Building the right team of strategic partners is key to effectively completing development/new construction projects.

." -...this doesn't seem to be the case where the state building code wasn't adopted.