Look online to see if any properties are being offered for sale or rent with a legal basement unit and you may get a tip off.

I typically use regional banks to fund my deals but my 2 go-to banks have either slowed or fully stopped investment loans so recently I've connected with several online brokers but all came from ads or they reached out to me vs personal recommendations and intros so I have concerns. 

This is not something you will get online you need an attorney to draft documents and it is just a matter of whether you draft them or their attorney drafts them

The downstairs tenant has sent me videos of the noise, and it seems excessively loud and obnoxious. 

Send a letter giving them your/pm's contact and a detailed explanation on how to pay the rent (Helpful with older residents if you are going from check to online billing.

Lots of material online.  

I also did some investigating online and see the renter has a tendency not to hold down a job.I offered asking price (fair market) because it was the sellers bottom line and the fact there were already tenants renting.

I think you are answering your own question here - if you can't find reviews or much info online then....

I agree with the above that Quickbooks Online (QBO) will work for you.

The address was submitted to their red book and now shows up online as official address when I try to ship items from Amazon or similar websites.