The Deck Editor is shipped together with Wiz Solitaire. When you download and mount the dmg archive, you'll find these files:
WizSolitaire-*.dmg contents
Typically, you should drag'n'drop the whole WizSolitaire folder into your Applications directory. In this way you'll also install the Space deck, which you can use as an alternative to the Standard one (other decks are available for download).
Yes: you can use the New Game with Options... item under the Game menu to choose how many decks you wish to play with.
You don't need to do it every time you start a new game, since it will remember the last used setting.
The same menu item can be used to set the options for other games as well (for instance, in Klondike you can use it to change the number of cards dealt when clicking on the deck or to allow partial column moves).
Unless specified otherwise, our general policy is that you can freely upgrade to any new minor release of the products you've bought.
For instance, if you've bought Wiz Solitaire 1.24, you're free to upgrade to 1.25, 1.26 and so on. When/if version 2.0 will be released, you will probably need to get a new license.
Version numbers used by the products on this site are not decimals*: they follow the scheme <version>.<revision>
Thus, 2.10 is not the same as 2.1: the latest MP3 Cat is version 2, revision 10. The previous one was actually 2.9.
* it would be too boring if everyone followed this rule: some people really use them as they were decimals or even marketing tools There's a quite long article on several versioning schemes on Wikipedia for the most curious ones.