But since that is a huge PIA, you probably won't want to do all that.There are also products designed to go in between subfloor and hardwood floor, like this:  https://acousticalsolutions.com/product/iso-step-floor-underlayment/And of course if you are considering ripping out the subfloors, you could go with a thick wall-to-wall carpet on top of a thick pad instead of hardwood.

A dollar bill under and old carpet, dated around 1950s.   

@Patrick Strain on the carpet , the corners looks dark and the dark patches in the middle.. could be water damage/mold?

If the unit has carpeting, replacing that may be a good idea as well.

Buy a mom and pop house on the mls cheaper, throw in some new paint and carpet and you’re good.

BRAND NEW appliances and carpeting.

If you attribute some of the purchase price to things like furniture, carpet, appliances and other assets that fall in the shorter useful lives you can get the benefit much faster (and sometimes immediately!).

Vinyl flooring or carpet in bedrooms.

The home was a custom build including custom interior paint and carpet that included one bedroom with bright pink walls and purple carpet, another bedroom with bright blue walls and green carpet, and the interior of the garage painted red.

My guess is they are not super cheap (cheap cabinets, Formica countertops, roll vinyl or cheap carpet, one piece tub/showers, etc) that will be required to keep your costs super low.While I personally purchase all my own material for my builds, its not as easy as you think.