Definitely start with 10% to see how your skin reacts. As for brand, I like Vivier or PSF but my skin is combination dry/normal. People like Cellular Skin Rx, Skinceuticals and Cellex C but I found that they dried out my skin.
Regardless of the brand, you definitely want the C to be in a serum because it has to be stable. You also need to keep it at a good temp and sealed well because it will oxidize quickly. To get the best information, I would check out that thread I linked to above from essential day spa. Those guys know their stuff, especially when it comes to Vitamin C.
Finally - for a really great product but a bit more costly, I would try Skinceuticals C with ferulic acid. That is supposed to be amazing.
I have a discount code for skinstore.com, they have some brands but not PSF or Vivier. Maybe just Murad and Skinceuticals. Let me know where you order from!