Welcome to the fret. I would recommend that you play the guitar, as you received it, for a couple of weeks. Of course, if it has some major setup issues, I would take care of those, but unless something is broken, I'd leave it alone at first.
Now, the trem block does contribute greatly to a strat's sustain. Material density here can contribute to increased sustain, and that is the main advantage to using something like a Callaham block.
In case you're interested, here's Callaham's URL.
Callaham's Made in Mexico Upgrades
Callham Home Page
The pickguard can be either very easy, or a major pain. First, not all guitars are created equal. In the past, I've had everything from pickguards that were a drop-in, with no modifications needed, to others that didn't even cover the entire cavity, so this is an area where it is best to have either the stock guard, or a tracing handy, to match up with potential replacements. As an example, a while back, I bought a pickguard for my P Bass. The bass' body was a USA '84 part. I got a pickguard, shielded it all out in copper tape, and then discovered that it was too short down at the end, by the controls. It turns out that 1984 was a major year of transition for Fender (move to Corona, sale of company, etc), and CBS was using up all the miscelaneous parts that were laying around at the Fullerton plant, so a number of instruments were built that didn't exactly match the sales brochures or engineering drawings, and my Precision's body is just one example. Squier strats and teles are known for inconsistencies when it comes to parts. I wanted to replace the pickguard on my daughter's Squier strat (Affinity), and I had to do some hunting to come up with one that matched pretty close. Even at that, I had to drill a few holes, and in one case, fill a hole and re-drill right next to it.
Pickguards...
More pickguards...
And even more pickguards
As time goes by, you may find that you want to replace the pickups. My current strat favorite is Rio Grande's "Muy Grande" pickups. Hot, but with plenty of definition, they're great for that "Stevie" sound.
Rio Grande Pickups
