1. Roof is inoperative as are all 4 windows. Could be a manual reset, and a fuse for the windows, or regulators and roof motor, from free - £££
2. Lacquer peeling on offside rear quarter. - Will need a respray
3. Front wings have been poorly resprayed. - Will need a respray, you could plasti dip it or have it wrapped, £600-£2000
4. Leather seats pretty worn. - The side bolster repair is £50-£100, then clean and redye the seats, that would cost about £500 professionally, £100 if done yourself and buying all chemicals. So £150 - £600 depending on the route you take
You're talking £750 - £3000 depending on if you do the work yourself and the roof is free, if roof isn't free add £100-£500 on top mate.
I'm selling mine though, and yeah it's a bit of a shameless plug but my roof works perfectly, it's just been resprayed with plastidip (including lacquer), all the electronics work too!
Here's the link
http://www.ebay.co.uk/itm/like/Audi-A4-Cabriolet-Convertible-full-service-history-well-looked-after/112292734500?hlpht=true&ops=true&viphx=1&_trksid=p5197.c100068.m2280&_trkparms=ao=1&asc=20140211130857&meid=a9ee539dba1d48f49b4200794db12107&pid=100068&clkid=2390739529947633609&_qi=RTM2063723