I'm looking along the lines of plain stained concrete, $3 psf tile/carpet etc and nice box store cabinets not a glitzy custom project.

@Rome Xu, yup you can buy a 4 unit residential property with a plain boring 30 year fixed residential mortgage.20% down if you're going to live there. 25% down if you aren't. 

Owner is sick and can't keep them.

My particular case is that I am sick of traveling for my job and missing out on my children growing up.

@Wenhao Leu, I do plain 30 year fixed loans using combinations of subject property rental income, asset-depletion-as-income, and helping folks like you get honest with the IRS for a year, on a fairly regular basis.

They unfortunately have a sick family member, and that is likely a growing cost for them.  

I set clear expectations early on that rent is always due on or before the 1st of the month.In my lease, I give a 5 day grace period -- I tell my tenants this is offered as a courtesy and should be used sparingly (ie birth of a new child, death of a loved one, sickness, etc).

The answer is no,plain and simple.If he wants to run a home based business so badly,he can buy his own house and use his own insurance and take his own chances with city code enforcement.This is a legal disaster waiting to happen if you allow it.Find better tenants if they threaten to move,you'll sleep easier at night.

You will get sick of constantly cleaning and replacing carpet so that your initial savings will probably disappear.  

In case your building is a very small building, install a lock box in hallways, a place that is a bit protected of the weather and not being in plain view.