As far as budget Standard decks go, the suggestion you found is probably the best one. Buying Match Strike twice will give you most of the cards you need to build a Standard Grachomp deck, as well as a lot of important staple cards that can be used in other decks (such as 4 copies of Cynthia).
As you mention, Blazing Volcano is currently the best deck in the Theme format. However, the same trick of buying the deck twice won't work as well, the reason being that Garchomp is significantly more viable in Standard than Blaziken. The problem is that Blaziken simply doesn't do enough damage. You'll never OHKO anything, and it doesn't do enough spread damage to justify using a Stage 2 that needs 3 energies to attack (Tapu Koko does the same thing for a single energy attachment). Garchomp, on the other hand, can potentially do 200+ damage for the same energy cost as Blaziken. Besides that, Lucario is better than Magcargo (assuming that you can keep a Garchomp in play at all times).
My recommendation to you is that you buy all three. Buy Mach Strike twice to build a Standard Garchomp deck, and buy only one copy of Blazing Volcano to play in the Theme format.
As to your question for the rotation, here's how it works. Every few months, a new set is released. The last set to be released was Lost Thunder, and the next one that's coming out soon is Team Up. Once a year, a few of the older sets rotate out of Standard. For example, as of the last rotation, all the sets older than the Sun and Moon Base Set have rotated into Expanded. The next time (somewhere in June/July, forgot the exact month), a portion of the older Sun and Moon sets will rotate out.
Currently, all the cards that have been printed in the SM Era are available in Standard. Besides that, any of the older cards that have had a reprint in a Standard set are also usable (for example, I'm allowed to use an old HGSS Double Colorless Energy because it has also been printed in the SM sets).
All of the cards you obtain from buying SM Era Theme Decks are currently playable in Standard, but if you decide to buy and older Theme Deck (such as Dark Hammer from the XY Era), most of those cards will be Expanded only (reprinted cards will still be usable).
Edited by Sakura150612, 27 January 2019 - 01:14 PM.