What screens do you do, or not do?

First off- $4.5M for 48 units and 50% are vacant is not a steal, unless this property is located in a downtown area and you have a contact with a developer who will pay 2X to build a high rise. $93k/unit for 50% vacancy doesn't carry hardly any debt, hence why they are capping you at $500k.

I personally started investing there in 2019 with a budget not too far off from yours, and I've been able to build up a portfolio of 12 doors since then, which now kicks off over $16,500 in gross monthly rents.Detroit has a lot going for it—affordable price points, strong rental demand, and good opportunities for cash flow, especially with single-family homes and duplexes in certain areas.

Egress: Two methods of egress, typically a door and a window.Ceiling height: At least half of the room should have a ceiling height of 7 feet.

It is usually much easier to seal a existing door way then it is to move walls in my experience. 

Interest on a HELOC is adjustable, if interest rates rise you are screwed.For you folks that weren't in real estate in 2003-2006 and when the market started collapsing in 2007.

Rising home prices for developers isn't new, but if you were in contract, what they did is most likely illegal.

There can also be chemicals in the treated lumber that are harmful to the environment.Invest in a good screen or built in door system.  

The roof is over the next door properties property line.

There are many horror stories on collecting for damages from OTAs including a current thread on BP where guest left doors and windows open upon exit that resulted in frozen pipes.