how much rent should I expect to pay?
depends what and where... size and location...
there are loads of site with places for rent - try gumtree.pl to give an idea
Where are the nice areas to live?
quite a few... what do you mean by nice?
Costs of living eg food, travel, etc?
depends on your lifestyle but it can be as cheap or expensive as you want...
What fun things can I get up to?
no shortage of fun places to hang out... did you look at warsaw-life.com ...?
Will my Polish workmates resent me moving to their country