I am looking to invest out of state into small multi family and would like to build relationships with mentors and investors alike.

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

It's written out in the Deed of Trust/Mortgage and just follow the agreement.

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?

Build the relationship with them.

If you are doing a bunch of STR builds, then you really need a CPA to guide you through this.

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. 

When you have your first issue, I'd have a trusted plumber (reach out if you want a good one) that can come same day to make the repair and can quote the replacement in its entirety; that person will usually give you the best pricing given that they're already there.