As the others have said, the biggest drop in prices were about 6 months ago when people were desperate to sell due to the economic climate. Things have stablised now so expecing another big price fall is being unfair to the market, so just get the best you can afford now and be happy with it. The enjoyment far outweighs the cost
As for engine choice, you do your research and you make your choice. The only difference I can see is that the torque curve has changed to make more headline BHP over low down torque. The 80% changed engine is just marketing. Yes bits have changed, but only slightly and they didnt yeild a lot of gain in real terms. If you want more power, you be best off getting an 03 and remapping it with the money you saved, would be far more drivable power than an 07+. Saying that, the interior is nicer in the 07+. But the tax is more. So you make the choice on whats important to you and buy one