1.)Kent Armstrong and Mighty Mite pickups are good cheaper makes generally- talk to the tech at your local gtr shop to see if you can cajole him into letting you use a guitar with some replacement pickups on it.

2.)No it's not difficult to change pickups - usually the black wire goes to the back of the volume pot and the white one to the selector - but check with fender/squier for a wiring diagram,or make your own by noting down where it all went before its generally just a matter of replacing wires like for like. Do make sure that you get the middle pickup in the right place as it's wound the other way to the other two so it gives you a "quack" sound in positions 2 and 4.

3.)Kent Armstrong(and others) make stacked humbuckers which are the same size as single coils (God knows how they squeeze 2 decent sized coils on top of each other) and should fit straight in.

I dont know how well you can trust the reviews anymore but http://www.harmony-central.com/Guitar/Data2/ has pickup reviews.

Good luck, I reckon if Squiers had better pickups we wouldn't need to buy mid priced Fenders..