They are trashy and dirty and fill the house with so much stuff the plumber couldn't even get to the kitchen sink to fix a plumbing problem they caused.

The bad news is it'll need new flooring, kitchen and bathroom as well as doors and windows.

I would like to renovate the kitchen, repaint, and furnish and equip the unit, so I am setting my max budget at $50k.  

A few items in the kitchen and bath...maybe something on the mantle...all tastefully done....it can be very effective. 

(For example, many investors talk of doing rehab on a 50k budget--but then on the same project, many GCs will charge that much for the kitchen alone.)Is it because the former buy materials themselves & have subcontractors install them?

The landlords have finally accepted defeat and have told my daughter that they need to demo the kitchen and add the studio to the adjacent 4BDR unit.  

we used the extra blown in insulation to fill in the crawlspace under the kitchen and under the bathroom.I used the foam in a can to seal all the light mounts and smoke detector mounts with firestop and I am happy to say the house is more quiet and the heat pump does not struggle anymore! 

I have since done a complete rehab with new kitchen and bathroom, high end carpet in the bed rooms, porcelain tile in all other house space, deck install and all new appliances.

For example, I imagine I could go as far as creating an entirely separate kitchen and bathroom for the studio unit and its own exterior door, but could I do as little as locking off the living room as separate quarters with the existing living spaces, kitchen and bathrooms as shared areas? 

The kitchen is in the middle of the first floor enabling it to easily service both front areas and back yard.