I'll restate a few things here.
1. It all depends on what style you want. Each artist has their forte and what they accel at best.
2. Be careful of artists doing work in 512x512. It may look amazing in the preview, but will be over 16 times lesser quality ingame.
3. 1 million is overkill. afaik, I have been the only texture maker to have completed a 1 million TC request, and even I think that the set I made was not 1M quality. There is no way to achieve 1M quality. Once you get past 300k, you're pretty much paying for publicity and attention. It won't buy you better quality, but it will draw in more interest.
People of interest to you in this endevor:
Avwave, Hanz0, Mrama, BenDover and the rest of the TTM.