Is there a certain cost per unit I can expect to pay based on the condition of the building? 

Any multifamily 2-4 unit can be purchased under FHA with 3.5%, VA (zero down), Conventional 20%-25% down primary for primary 2-4 units depending on # of units.

My girlfriend and I are looking to move to Chicago some time in 2025 (likely the fall) for her job, and we are looking to buy a small multifamily property (2-4 units).

This strategy starts to compound when you find homes that have an ADU (accessory dwelling unit, casita, bungalow, etc…) along with the main house.

We are looking for 15 dwelling units to 25 dwelling units per acre in a suburban area.

But the property is a duplex and so there's no way to know which unit is doing the damage.

HI John, You can always start off by house hacking meaning you live in one UNIT or ROOM and renting out the other vacant units/rooms to offset your monthly mortgage payment.There are several ways to run and make sure you are maximizing your rental income while keeping your living expense as low as possible.

I calculate around 90k-120k per Multifamily unit.You can develop a strategy and a model unit and continue to make that same unit for the next projects.

If I wanted to owner-occupy a 5-10 unit apartment building, would I be able to do 5% down conventional financing or must I do the 20% down simply because anything 5+ units is commercial? 

Just a reminder that even if he owes you the entire security deposit, you need to follow your state and local laws regarding notice to him showing itemized deductions from security deposit, any balance left or balance due to you, and proof you sent to last known address, I believe, even if it's your own unit, as he could have put a forwarding notice in with USPS.