The purpose of the multiple levels is to keep the surface as smooth as possible.

@Andre KeyThe commercial air scrubber linked in the previous post will be what you need- BUT, in addition, you will need to clean the A/C coil and probably replace carpet, duct work - cleaning won't help - I had a smoked house and we re-painted every surface/primed with kilz, replaced carpet and cleaned coil and ducts--- smell was improved, but not gone until we replaced ducts- The air scrubber is a great starting point with a new project - it will remove 90% of the smell before you even paint-Good luck

Now if you're thinking about all the Mom and Pop outfits out there trying to eek out a piece of the pie for themselves with antiquated, expensive and very limiting technologies then I couldn't agree more.In the early onset, as with most major growth industries, you will see a growth curve of small start ups cottage industry businesses surfacing and perhaps even sustaining themselves or even evolving into one of the major players. 

When I reached my hand in the hole, I found a 2" 'channel' about 12-16" below the surface just Pouring water into my fence post hole!! 

To fill voids due to uneven surfaces so that the backerboard will not have deflection and bounce if the fastening system is not 100% capable of eliminating movement.

Your seller has some life issue where his finances become known, divorce, bankruptcy, he becomes incapacitated, he dies, he goes to an attorney to buy another home and he mentions it, his taxes are audited, he tries to refinance his mortgage and it surfaces as an asset, he attempts to qualify for any government entitlement or benefit, he talks to a Realtor, he hears about selling notes for cash and attempts to do so.....I could go on, point is, you can not hide what you did, it will surface someday.Statutes of limitations vary, but fraud is universal in that the limitation begins upon the discovery of the fraud.Welcome, "you can check out but you can never leave"......without paying:)

A few telltale clues incline me to believe that it's original construction and if so, the surface underneath is likely to be 1/2" drywall (as opposed to an older home with plaster).You can check for sure by simply removing an electrical plug or switch face plate and looking at the wall material.As @Russell Brazil pointed out, when overlaying the walls, there is a lot to consider in how the additional thickness impacts the existing trim.  

We had to replace all floor coverings (all rooms had cigarette burns and permeated smoke odor into the carpet and pad, had to clean-seal-repaint all walls and ceilings, deep clean all cabinetry and every surface in the unit.  

If its the same (usually cheaper), I would look into Corian, I'm particular to sandstone because it hides damage well and if there are scratches you can just sand them out and you have a brand new surface, then if there are major problems they can typically fuse in new material.

I just wanted to point out that we don't have enough info on the numbers to judge, even though on the surface it sounds high (74%).