Differences between 530d & 535d M Sport

[Remster500]

I'm in the market for a 535d M Sport Estate (2005-2006) and have been watching the classifieds on here and Autotrader. I have a budget of £30k and can't help noticing that the next model down, i.e. 530d M Sport, commands a much lower price.

Can anyone explain the main differences between the 2 models to a BMW newbie? Is it simply engine? Can a 530d be re-mapped to give similar figures to that of it's big brother 535d?

Also, what are deemed the "must have options" on these models?

Thanks for any help you can offer.