As Sebi has already mentioned, the output of pickups varies greatly, and active ones will generally have much higher output than passive ones.
Usually guitar amplifiers have fairly high input impedances, often as high as 1 megaohm, but I've used 100K quite happily in the past.
Most guitars can put out a great deal more than 20mV, some (played hard) can reach 1V or so.
Any guitar circuits you find are likely to be designed to work with a guitar, so it doesn't really matter a great deal - you simply adjust the controls on the guitar, and the way you play, for the sound you want.