SLK Brabus can mean anything from SLK200 I4 with Brabus wheels and shift knob to SLK 6.5 v8 Monster...huge huge difference on the level of mods that Brabus could have done and still sold the car with a Brabus badge.
My mistake... it's a 2005 SLK 55 AMG "Powered by Brabus" B55. THe cosmetic items I notice are the engine cover, tinted tail lights, seats look different, Brabus door sills. The owner says it produces 396 HP (not really a huge gain from the stock 355).
Brabus is one of the most reliable tuners for MBs, but that's all relative! how familiar are you with tuned cars?
Make sure you get full paperwork and receipts, and then crosscheck with Brabus to confirm. There are a lot of 'fake' Brabus's floating around, and few approved shops, so even if you trust the owner, he/she may have been scammed as well.