Hi, welcome to the forum.
If you're looking for 2010 to 2013 that will be mostly Mk6, or an early Mk7 if built very late 2012 onwards.
As for which is most economical, this depends on how it is driven. The smaller petrol engines are best around town, although if they are frequently worked hard (e.g. lots of miles at motorway speeds) you may be better off with a diesel.
Bluemotion is a model in its own right with fitted with a 1.6 diesel engine, but also some of the engines in other trim levels are referred to as Bluemotion Technology. It's important not to be confused by this. The standalone Bluemotion model is geared towards economy, but isn't particularly quick. Nor is it particularly well equipped as standard.
If you're thinking about a diesel engine, be aware that these will have a diesel particulate filter fitted, which may be prone to blockages if you drive lots of miles around town at low revs. They need to be driven periodically at high revs to burn off the particulates, otherwise you'll need to pay your local garage to unblock it for you, which may be expensive. If fuel economy is important to you, it would probably be best to avoid the 2.0 litre petrol engines.
