All units need interior remodeling/components: flooring, kitchen, baths, paint....likely about $25,000 per unit to have a finished product.

He gave me a quote (see below) for $6,000 - $25,000 depending on how good a fix I want with a summary of the problem as follows: "The structure has settled...the measurements indicate a drop that ranges from -1.9" to -0.4", with the average drop at 1.23"...the structure has sustained two severe foundation cracks...there is wall cracking on the inside of the home: cracks in the front room and cracks in the kitchen."

However the unit had a living room and a dining room, plus a kitchen big enough for a dining area, so we put french doors up to turn the living room into a 2nd bedroom, and got $600/mo rent up there!

welcome and ur titles says it all: u need/want a duplex. unless ur also open to just getting a SFH and renting out some of the bedrooms and sharing kitchen and possibly bathroom(s).good luck wheeling and dealing in ur local turf; ur starting so young so dont rush - dont overpay for ur investment. maybe look at HUD and REO deals!

After learning that one of my tenants was pouring grease down the kitchen sink (I thought everyone knew not to do that!)

Having to flip two kitchens instead of one, double the bathrooms and appliances...

This was the kitchen during demo. 

Also if someone can help me with these questions :1) who is responsible for kitchen ventilation fan, the landlord or the tenant?

I have run the numbers on the house and if I can get it at the right price and make the appropriate improvements (new roof, new kitchen, add second bath) it will produce a nice profit.

I'm mostly looking for paint/floors maybe some kitchen attention and minor bathroom updates.