If you look on the Dragon Coldfire post then you can save money on the mediator by spending a lot more on an expansion board with a Coldfire CPU which will still allow it to be a "Classic Amiga" although if you go down the Shark PPC and mediator route you'll still need an accelerator for the Mediator to work so you can use the Shark.
Confused?? Unfortunatly this is due to the neglect the Amiga has suffered and all the 3 party developers who think that speciallised electronics is a good idea. To be honest a Graphics card, IDE interface and tower are needed if you want to carry on with your Amiga and they do bring life to it and to have a graphics card you have to go down the Mediator route as the Zorro Graphics cards were terrible even when they came out. Unfortunatly the Amiga is a Specialist Hobby computer now and that means expensive.