I've been running LM reps for the past year or so:
They are 8.5x19 running 225/35/19 all round. The offset they come in is ET45 and for the fronts to clear the brake caliper you need a set of 17.5mm spacers. This is the minimum width spacer you can use as the calipers are huge.
I went for 17.5mm spacers all round to even it out, though as I've got it dropped low on coilovers, I've actually got some 15mm spacers for the rear ready to go on. This should give it a few extra mm clearance as it rubs when I have four people in the car at the back.
I got the spacers imported from the US from ECS Tuning (
http://www.ecstuning.com/Audi-B7_A4-Quattro-2.0T/Wheels/Spacers/ES119/) You will also need longer wheel bolts which ECS sell - if you look through their site, you'll find them. You'll want conical bolts to fit the LM reps. Drop them an email before you order & they'll help you out with the right length, or search online for the OEM length & add 17.5mm.
I suggest you lower your car as well as 19s look a bit silly even on the S-Line/S4 suspension. I got my coilovers fitted first and they're FK Konigsports.
I got the wheels via eBay, fitted with tyres & balanced ready to go. Most of them are listed with 235/35/19 tyres but for clearance, especially if you are planning to lower the car, I'd reccomend going for 225s. If you drop the seller a message, they should do the package for the same price - in fact, 225/35/19 are a bit cheaper generally.
Here are a few sellers I've found for you... with a few different colour options too:
http://cgi.ebay.co.uk/19-BBS-LM-ALL...s_tyres_Trims_Car_Rims_ET?hash=item1c0d3680ec
http://cgi.ebay.co.uk/19-BBS-LM-ALL...s_tyres_Trims_Car_Rims_ET?hash=item1c0ad1ca07
http://cgi.ebay.co.uk/19-BBS-LM-ALL...s_tyres_Trims_Car_Rims_ET?hash=item1c0a3fc0cd
Hope this helps!
Cheers,
Matt.