You are close.

Scans in the 3D printing realm are digital copies of existing physical miniatures. They are incredibly difficult to find and acquire for obvious reasons and vary in quality.

No. The collection is the most comprehensive you'll likely ever find but it isn't strictly complete. Some miniatures or ranges just do not have scans that exist to-date.

You are strictly paying for knowledge that will lead to what you are after and is eluded to. I am almost certain that if you are remotely interested in scans this will be a fruitful venture. 

No. For obvious reasons this is a leap of faith kind of a arrangement. 

Yes. Essentially, this collection boils down to large collection of digital files. What you choose to do thereafter is your business. You'll have to exercise standard printing procedures such as generating supports on your software of choice, slicing etc. Very easy, very common.

The quality of the files varies depending on who originally produced them etc. This is a vast collection so has many authors.

Slicing software blowing up the size of miniatures can be jarring at first because your eye can pick up imperfections on a monitor but in practice, a good quality printer and resin - the results will be speak for themselves. When a file is printed at its correct scale and becomes a physical object, quality primer and paint job later and most projects would be indistinguishable from the real thing. 

This is as good as it gets and you'll be very pleased to have all these options at hand.

This is the kind of thing that is worth beginning your printing journey if you have yet to start.

Yes and No. The best and most successful application of scans is for "The World that was"

This is for multiple reasons but to name afew; The originals availability, rarity, cost and sculpting detail lend itself to desire, scanning production and 1:1 results.

This has made the Fantasy component of this collection very compelling and rich with depth.

The Sci Fi component is equally vast and interesting but has more gaps in available scans for specific factions. This is normal, that universe is much larger in scale and scope and not every miniature in existence can be or has been scanned.