I have done mostly cosmetic changes throughout my years there(painting, fixtures, door replacements, flooring).

If you don't invest a lot into something, $100/door might be great.

Walk into the front door, go up stairs to the upper 2 units and down stairs to go to the lower 2 units.

Now im a bus driver. 2018 was my 1st year filing taxes

If I was smart enough to invest in real estate when I was single and just out of college, this would have been the perfect strategy to get my foot in the door.

I have 30 doors and need something that's cheap, customizable, and feature rich.

Paint the front door.

@Lane NelsonI knocked on the door after I bought my first house at auction, told them that I was the new owner and offered them $750 cash to move out at the end of the month.

With that said, it is important to understand the market drivers and the local economy if you are targeting smaller markets. 

I have heard some people setting up their heating cooling system to shutoff if a door or window is let open for a certain amount of time.You can also add a temperature offset to make it look like it is a few degrees cooler or warmer than actual since sometimes temp setting is a mental thing.