1) are the extras; xenon bulbs / 18" tyres etc going to kill me in running costs?
3) is the sline suspension / big wheels ride harsh?
4) is the sline worth the extra cost over the cheaper one?
1) Decent 17" tyres are £80 each / 18" £100
3) Fairly harsh, although S-Line trim could I believe be specced without "Sports Suspension" - depending on tyres 18" wheels "tram-line". Even the S-Line can be specc-ed with 17" wheels, which I think would take some of the harshness out of the ride.
4) Probably - as it's the little touches that you don't see on a spec sheet, extra door (puddle?) lights, 10 Speaker stereo vs whatever the norm is? Sline trim seats and Steering wheel.
SE and S-Line aren't the only options though - there is the SE Technik which has more extras (including Cruise Control - that isn't standard on any other model), and the Sport which has everything the S-Line has, barring the S-Line badging and different bumpers.
Not sure where you are looking, but worth looking at Audi Approved Used e.g.
A3 1.6 Diesel Sport - £15,990, 28.5k miles
http://www.audi.co.uk/used-cars/use...bedford/604856716-21257.aspx?srcmdc=se_na_re_
When I bought my A3, Audi (JCT600 Boston) were very helpful, as they fetched the only one in my price range, with the spec and colour I wanted - from Wales.