Try this device to measure the noise: https://www.amazon.com/Indicator-30-130dBA-Detector-Temperat...You can also stick the same device by windows and doors to see if temperature drop - which would indicate an insulation issue.In addition to getting an Energy Audit, you may also want to look into any state, local or utility programs that offer discounted or financed energy improvement programs.

As someone eager to learn and navigate the complexities of real estate investing, I believe your guidance could provide invaluable direction.I am particularly interested in understanding the best advice you received or discovered throughout your real estate investment journey.

For all of the additional work involved, I'd be hesitant to do a STR deal at 10-15% but of course it depends on what your goals are for the property and for your finances. 

Additionally, It is noteworthy that the seller was a member of the HOA board at the time of obtaining this permit, raising questions about the legitimacy of the process.Though I am not currently residing in the apartment, I took the initiative to rectify the flooring issue in cooperation with the HOA, given the significant disturbance experienced by the current residents due to the subpar quality of the previous labor.Furthermore, short after I bought the unit, I discovered an assessment imposed on the condominium, which I was not informed about prior to purchasing the unit.

You may have other non-financial costs associated with this type of property that compensate for the greater cash flow (e.g. additional risk, needy tenants, tenants whose priorities do not leave room for taking care of your property, etc.). 

The longer you wait, the more likely mold starts to form and creates additional expenses and damage for your project.

Should I be doing anything additional knowing he is running a business out of my house?  

He backed off.My insurance agent advised me in the future to have myself named as an additional insured on the renter's policy so I will be notified when the policy got cancelled.

If I only receive the down payment this year (let's say 20%) can I write off the entire 250k towards that and structure future years to try to get in lower LTCG categories than all at once?

I recently reviewed a buyer's agreement which stated 3% on gross selling price, plus $695 flat fee in addition to percentage, for services provided.