If it localize, i.e. a roof leak in a certain area, a wall in a basement that have moisture coming through.

As a vote of confidence some foreign firms (Akelius for example) have been penetrating the market Montreal/Toronto, buying buildings at incredibly high prices.  

If you think about it, there is no way that anything that doesn't penetrate as far as the urine did can possibly remove all the smell, because it simply doesn't get far enough to remove it!

This is NOT a DIY project...You'll may need to remove the sheetrock and flooring, use an anti-microbial to kill the existing mold, use dehumidification to dry out the area, scrub/shave the studs to remove spores (if the mold has penetrated the studs), etc.The costs could be anywhere from $2000-10,000 in my experience, and each contractor will probably give you a different idea of what needs to be done.

One home with a dog hadn't changed their filters during the winter; the unit worked harder and actually had moisture in the return because of the temperature differences. 

Read your contract through, as many standard contracts do have built-in protections that if termite/moisture or lender-required repairs are found, seller agrees to cover up to x% (and it usually states, if no number entered, parties agree to 1% of purchase price). 

Many have ice already, but if they are only making $300/mo from their current vendor and you are offering $600 and in a way that the clerk doesn't need to be involved (pay at counter), you could start getting some market penetration (this is similar to the private vending machine business).

Turns out the builders buttoned up the walls during a storm while there was significant moisture present and now the insides of the walls are full of mold.

Now they've decided to go to this Coretec product at $3.2/sqft that supposedly is fully waterproof, needs no moisture barrier, can go in kitchens and bathrooms, floating floor no glue required.

Older home have the potential for higher repair costs due to outdated electrical, plumbing, etc, but as long as there's no moisture issues these houses were built 10x better than today's houses.