An estoppel certificate is a form you have the tenant sign that confirms their lease start date, the amount of their security deposit, and other things.

I look at the tenant's financial situation (income security) as a very important part of the property analysis. 

This needs to be highlighted in the lease and acknowledged by them.Security Deposits: You can’t use their security deposit for routine cleaning, but if they leave things in bad shape beyond normal wear and tear, it’s fair game to deduct cleaning costs.I’d start by having a straightforward talk with them about the importance of keeping things clean for health reasons and property preservation.

Consider remote monitoring tools (like security cameras), and budget an extra 10-15% for surprises.

Consider a cash-out refinance or a small business loan to fund these repairs and secure the property long-term.

If you are going to go with secured mailboxes you will need to contact your local postal office and tell them you have two new boxes that you will need to get "Arrow" locks installed on.

I have prospective tenants who signed a lease on 31st of October and paid security deposit same day.

Given your experience and strong financials, you have some solid options:Commercial or Portfolio Loan: These are tailored for investors and can help you avoid personal guarantees, especially since you have an LLC.BRRRR Loan (Buy, Rehab, Rent, Refinance, Repeat): Some lenders offer BRRRR-specific loans with funding for purchase and rehab, allowing you to refinance based on the new ARV once stabilized.HELOC or Cash-Out Refi on Current Rentals: Tap into the equity on your single-family rentals to secure funding for the 4-plex purchase and rehab without high origination fees.Each has pros depending on your cash flow goals and timeline.

But all of these investments operate under highly regulated and constrained exemptions to securities law, and I have been told it is very difficult to do the above legally.

I don't know how IL laws are but I think in most places a local municipality can only charge a fee to provide a service and the cost of the fee must roughly correspond to the work involved in providing the service.