I would think it depends how much money you have to devote to a collection, and what sorts of things you wish to collect within the Cars world.

I know as a kid, I didn't "collect" things, but I still had collections, and now all of them are long gone.  Some of them I have gone back and bought to regain them, but that's just me.

If you really want to collect Cars, you might start with the 2014 releases, then get the 2015 as they are released, and slowly work your way back and collect 2013, 2012, etc. as you have time and money.

For your mother, I'll point out that a properly cared for collection (any collection) can become a significant investment, but it also helps a young person give focus to their life, gives them goals and responsibilities, and can lead them to a life that is more well-rounded, educated, and whole.

A "Desert set" in good condition will likely hold its value, as long as collectors will still want Cars going into the future.  With more Toons coming, as well as a third Cars and Planes movie, I suspect collecting will be hot for at least another ten to fifteen years.  More importantly, you also need to enjoy them for what they are — not just as an investment.  Have fun with your collecting, or else then it is just a chore.

I'm an opener for that reason, I like handling the Cars, displaying them, etc., and for me, the packaging has very little value — though I do understand the thought process behind collecting them un-opened.

I started after Cars with the Desert series, and then dropped out for a few years — now I have over 800 unique Cars. It is addicting.