Okay so coming from an Impreza you won't be scared filling it very often. The 3.0 TDI's are thirsty - anything from around 18-24mpg around town and anything from 32-37/8 on a long run. I've seen 41.4mpg on mine a small number of times and MarkTM on the forum has touched 50mpg.
Re gearboxes, a quattro automatic will be tiptronic and any fwd auto will be multitronic. The former is a better, more reliable gearbox as the multitronics can be troublesome with control modules and are expensive to replace. From about 2009 onwards they became significantly better but I still wouldn't trust one personally plus I don't like the way they drive.
You've already seen the brochure for the C6 so that covers most options and what the Le Mans comes with above the S-Line (parking sensors, leather, interior light pack, 19s)
Re road tax, there was a point in around 2006/7 ('07 plates really) where they lowered the Co2 by 1 or 2g/km which put the car into the lower(!) band of £280/year. Some older cars oddly were in the bracket of £480 I think it is.
quattro will chew cheap tyres but will love premium rubber. I have Michelin PS2 (255/35/19) on mine which have just ticked over 37k on the rear, the fronts needed replaced at 32k due to bad alignment on the car but they still had 2mm left. The rears will need done in the next few weeks.
I've had my car since 2009 when it was 1 year old and 33k on the clock. It now has 87k-ish and I recently created a thread here showing running costs -
Running Costs .. 2009-Present
Generally go as new as you can afford to avoid any early production issues. My '57 plate has been pretty faultless apart from the odd glowplug and intake manifold runner, there is a fix kit for this however so don't be too worried.
Remember there are loads of cars out there, so just keep looking and don't be afraid to travel for the right car. I travelled to Teesside to get mine and drove it 400 miles home where I managed an average MPG of high 20's
