What features should I look for in good running shoes?

I’m planning on running a marathon next year, and wondering what features I should be looking for in a pair of running shoes to best suit the role, e.g. flexibility, materials?

I want my feet to be comfortable, even while running on hard surfaces like tarmac and concrete.

Price isn’t really a concern – I’d be willing to pay upwards of 100 euro, or 140 dollars for a decent pair of shoes.