Having an LLC is not necessary.However, it should be decided once you have a conversation with an Attorney and an Insurance agent.Some factors that come into play are1) Your Net worth2) Your Risk Tolerance3) Your Risk Exposure

Adequate insurance is generally recommended as a first line of defense and may play a part in this decision as well. 

I have no clue if she talked to any buyers besides myself at any point but without playing games I told her I could pay her what she asked for.

A lot of investors seem to be buying in here as long term defense plays.

Storage areas can include built-in cabinets, closets, attic space, and even outdoor sheds, etc.Laundry: Including a laundry area and machines is a big bonus for tenants, as they compare that with apartment living where they would have to use common laundry machines that require payment.HVAC: Make sure to include an effective dual heating & air-conditioning system to service the ADU, so your tenant is comfortable in the unit.Kitchen island: It may be difficult to squeeze in a kitchen island in a small ADU, but it’s a big bonus if you can make the space.

Mailers are long term plays, expensive, and have a low success rate.

Defintely talk to a lender because there are a ton of factors that will play into the amount of next mortgage you can get- the biggest ones I dealt with were DTI ratio and the monthly payment I would have to make on that HELOC.

I wasn't playing any music, I was sitting working on my computer and there was no noise. 

Unfortunately the building I bought in does not support STR but I still think its a solid play as a regular long term rental.

And it's completely up to you and the amount you are willing to spend.If you are planning on making real estate a long-term play then you should definitely be on the hunt for a good accountant and attorney.