I could probably help out a bit, but I've been super busy. 

For our STR we have a few different types of loans but I would always go with the 30 year fixed because you never know life gets busy and in 5 years a lot can change. 

I own a 3-unit multi-family with a business partner.

If the property is being sold for 400k (and I can afford the 20% down) but another 800k min needed for a complete renovation (my personal rough estimate), can I take out a traditional business loan to cover the rest, if I were to purchase in my LLC?

There comes a time that you just have to make a business decision about raising rents because owning real estate is expensive.

I would love to connect over coffee and learn more about your business!

We are in the "due diligence" phase of our evaluation and as business owners ourselves, we make decisions slowly and deliberately(over months/years). 

Since this would be considered your course of business because it is not a one off, you should also look into things like using 3rd party servicers etc. to maintain compliance.If you want to do it right, then yes use underwriters to review the loans and treat it as if you were Wells Fargo originating a loan to yourself.

This property must be used in a trade or business and held longer than 1 year.

So, that gives you some exposure..Its normally not advisable to mix business lines.