It usually starts with a thorough walkthrough of the property to list all repairs and improvements.  

A roof on the 600K property will cost more than on the 300K property, but you have more roofs to repair on the multiple 300K properties.

Even in the nice locations, most top out around $2,800.Of course there is also your increased insurance, vacancy, repairs, CapEx, and the never mentioned reduced rent on the main house since having a stranger living in your backyard usually isn't appealing to most renters. 

I learned this the hard way personally and had to get mine repaired.

A seller could also refuse to pay any concessions for buyers closing costs, appraisal required repairs, etc. but it would be to their own detriment.

If one or more units are vacant, the remaining units often don’t provide enough cash flow to cover expenses, making it harder to meet operating costs.Maintenance costs: A fourplex comes with four times the appliances, plumbing, HVAC systems, and other components to maintain, leading to significantly higher repair and maintenance expenses than single-family homes.If you want to see the detailed calculation, read this BP blog - More Units Doesn’t Mean More Money—Why a Single-Family Home Can Beat a Fourplex.Resale value: Multi-family properties have a limited buyer pool—mainly investors—who base their offers on CAP rates.

So there will be the cost of the inspections (not cheap) and the cost of the repairs.  

Prioritizing cash flow, mastering property management, and learning basic repairs can make a big difference.

My gross income is a bit less than $7k (and I'm renting near top of market rates) with minimal repairs ($2k/y), but...1.

Will they have trouble making the payments to you while also having enough money to keep up with the maintenance and repairs/ tenant issues/ all the expenses involved with owning the property?