Evidence to prove what? That he doped? It doesn't sound like that's at the core of the investigation. The real focus seems to be on fraud, illegal drug possession and sales, or racketeering. With that they won't be looking quite so much at 1999 tests, or the credibility of Landis testimony. They'll look hard at things like taxes, bank records, contracts, etc. Which happens to be the area of Novitzky's expertise.
Remember also this isn't entirely about Lance. I don't think investigators are aiming squarely at him, and I hope they aren't. He's sort of a middle fish here. A visible one certainly, and a bully, but still not likely the highest person on the pyramid. They'll be spending a lot of time looking at Weisel, maybe Stapleton, JB depending on how international this goes, and certainly some of the physicians and medical support. Alan Lim for example probably has a real hot seat waiting for him.