There are a number of sites available for surplus military rifles and specifically the AK-47. Any of them will have the information you want in detail with history, manufacturers configurations and markings, maintenance, diagrams and photos as well as the latest buyer reviews and prices.
kalashnikov.guns.ru/
ak-47.us/
ak-47.net/
surplusrifle.com/ak47/index.asp
forums.gunboards.com/
parallaxscurioandrelicfirearmsforums.yuku.com/bparallaxscurioandrelicfirearmsforums
warrifles.com/forums/index.phpThe versions of AKs, for example STG, WASR, CZ-58s do not always interchange parts with the actual Russian AK-47s. There are remanufactured AKs available for around $400 and modern versions of the AKs up to $1000. Also, along the Uganda, Sudan and Kenya borders the cost of an AK-47 assault rifle has dropped from 10 cows in 1986 to two cows in 2007
This is a decent enough FAQ for a buyers guide. Current prices may be different do a search.
ak-47.net/ak47/faq.html
Also check the latest laws, since the lift of weapons bans some things are available that weren't... unless you live under an oppresive government like the Demokratic Socialist State of California.