He received quotes to remove the texture ceiling, paint the interior, purchase and install new kitchen cabinets and counters (including completing the design), picked out 2 floorings to get quotes which will be provided this week, and determined furnace is beyond saving (I will likely get HVAC quote for him).  

We went all in on renovations—tore down a chimney to open up the space, put on a new roof, installed HVAC, updated floors, and completely redid the kitchen.

I usually recommend clients a pretty easy cosmetic rehab which is refinish floors, replace cabinets with modern white ones, add a white stone, new appliances, new bathroom vanity/mirror/light fixture and add in unit laundry. 

If that means taking out the trash, sweeping the floors, getting the coffee, whatever I will do it!

You could use a floor leveling product but check the square feet to see if the product covers large spaces.

But cat pee soaking into the floor is damage, failure to clean up liquids or other gross things, I'd rather not go into. 

BTW, if you have hardwood or even pine under the carpeting, you might consider refinishing the wood. 

Updated kitchen but I’ll need to do floors and update bathrooms.

It is set up like a duplex as there are 2br/1 bath on each floor.

It may limit attic space, however you should have plenty of room for Airhandlers, Waterheaters, etc. in the non-living area of the home on the ground floor.