My brother has owned a mex strat for a number of years and its playability is OK but the tone is weak and is better for a beginner. As a beginner guitar you are going to get beginner tones and that's not gonna change by changing out the pickups. The tone comes from the wood type used, the wiring, pickups, bridge and its always important but often overlooked but good tone also comes from good tuning pegs. The guitar has to stay in tune for there to be good tone.
I have a 62 Hot Rod Stratocaster that is the best guitar I have ever played. I bought a brand new Gibson Les Paul Studio recently that is total junk. It sounds like crap when I plug it in and it only gets worse as I play it. Gibson quality is not what it used to be apparently. Fender has done a great job with keeping the costs down for the consumer. If you want a quality guitar from Gibson you have to pay around $3000.00 for it. My Fender Strat is half that and is simply the best guitar I have played for 20 years.
My advice is spend a little more money and get a good quality Strat, but play all the strats your local guitar store has until you find the right one. A friend of mine bought the exact guitar I bought and his guitar doesn't have the same tone as my Strat, IMO.