1. Stock plates are about £105 and a basket is circa £80 i think. You may only need plates.
2. Nothing else needs replacing unless you spot something obviuosly worn. The drum possibly.
3. Pretty easy, especially if it's just a plate change. the basket is a little more involved but not difficult.
4. If it's just a plate change, 5mm and 4mm allen keys is all you need. If the basket needs replacing you'll need a holding tool, a 30/32mm socket and a big bar to get the drum off, and then a 13mm socket.
5. Half hour tops for a plate change. Comfortable hour to do the basket as well.
There are also performance options and longevity options on the plates and or basket. Top dollar is probably the 48T STM conversion at £280. Anywhere in between for the performance plates out on the market.
[Edited on 20-10-2005 by nelly]