This would include paining the walls, replacing vinyl floor with vinyl floor, replacing Formica counter top with Formica counter top. 

Always account for plumbing and electric, as if it's original, it's probably time for a copper/PVC upgrade that will last another 100 years. 

My main business currently is a printing company that specializes in screen printing, embroidery, vinyl sticker/decals, and anything marketing regarding small businesses, churches, etc. but want to switch over to real estate investing full time.

Later, I put in new vinyl windows and they have c/ha. 

When a section goes bad, you or your plumber can saw it out with a recipro saw with a diamond blade in it and replace it with section of PVC or ABS using Fernco couplings.

We found that they had let apartments go vacant for up to 6 months between tenants, they ordered 32 sets of window blinds for a unit that had only 8 windows, they put vinyl flooring into a unit that needed repair under the kitchen table and it was a different color from the rest of the vinyl in the kitchen. 

I just bought 17 boxes of heavy luxury vinyl flooring at Home Depot. 

Window cost depends on size; 30x54 six light vinyl frame with screen will cost ~ $200; install cost ~ $100.

It's usually easy enough to access the plumbing in the crawl space, so if you don't see PVC under there I'd strongly recommend considering replacing what is there. 

Invest in newer energy efficient vinyl windows.