1) Is it worth getting the american standard rather than the mexican?
Yes, because at the moment the quality is fantastic. I recently bought a 2004 Stratocaster from Mexico and it's one of the best guitars I've ever had (and I've played a lot, incl. several Strats). Even Squier (Standart, not Bullet or Affinity) are very good at the moment. They Squier Standart Strat and the Squier Telecaster Custom are by a mile the best budget guitars around nowadays. But then again...stick with the mexican since it's a Fender (warranty, re-sale value) and the parts are a bit better.
As for the differences Mexico/US strat: Mexico has a 3 or 5-piece body, US has a 2 or 3-piece body (talking standart, not custom shop). The necks are the same, both made in USA. The saddles and the finish are better on the US model. The tuners suck on both. (If you wanna change: Schaller locking tuners) Mexican models have ceramic magnet pickups, the US models have Alnico Single Coils. Both get the job done. If you want real good strat pickups, you'd have to get Fender Texas Specials (SRV, Gilmour, Knopfler) or Kinman Woodstocks (Hendrix) anyway.
2) Is it easy to change one of the pick-ups to a humbucker at a later stage, or do you have to go for the fat strat straight away (it's inevitable that I will buy a Les Paul at some point as well as my Strat).
If you want to buy a Les Paul anyway one day, but also want a Strat now, then get a real Stratocaster - that means one with 3 single coils, not a "fat" one.
You can change the bidge pickup for a humbucker in single coil format until you have your Les Paul. But you don't have to. With a good adjustment and a decent distorsion/overdrive, this might not even be neccesary. Use a wah to boost the mids, get a fuzz box...many ways lead to Rome.
The changing of pickups is quite easy. If you're unsure about it, just consult a music store near you. But any experienced guitarist should also be able to this.
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