1)Since servos have a much higher rpm range, and constant torque throughout that range, servos are usually geared to provide more torque, and higher resolution.
2) Those may be on the small side, but you'll also find that drives for them will eat up a lot of your budget, if not all of it. Rutex at the low end are $200 each. Anything else will be at least $400-$500 each, unless you get some Ebay deals. A few inexpensive drives are being developed right now, though.
HappyServo - Open Source Brushless Servo Controller
New sinusoidal AC servo drive development
3) Depend on what kind of price you can find the servos for. A set of Geckos and a power supply for DC brushed servos will be close to $500-$600 without the motors.
4) Not necessarily, but it really depends on the motors and drives.
Almost forgot, servo drives and amplifiers are usually the same thing.