If they're hoarders, they'll have to make sure that it's not cluttered, as code enforcement would otherwise create a problem for you.

When I have to change tenants I can easily delete the old tenants code and put in a new one.I thought this was a great idea and just wanted others opinion why this would not be a good idea or why you are not using this method.

Every inquiry on a credit report is coded.

The more difficult stuff is zip-code specific, but I think city-search is a good resource (if I'm remembering correctly).

The last line of the letter says "you are encouraged to immediately contact code enforcement to discuss resolution."

There is an exclusion for your primary residence which is found in sec 121 of the IRS code. 

You can exchange in and out of any state. 1031 Exchange is federal tax code. 

This will help you with your 1031 exchange "case" with the IRS. 1031 Exchanges are a federal IRS tax code instance and so anyone in the US can help with a 1031 exchange but @Dave Foster is one of the best!

It must meet the requirements per the building code IRC (I think Massachusetts follows a modified version of the IRC) you must provide the required amount of light, air/ vent, and egress.

If you do not then you can ask realtors, property managers, other investors as to what zip codes are the bad areas and stay away.