My 500K list needs to have good list hygiene applied to it before I begin sending to it. Here's what needs to be suppressed: spam traps, honey pots, static traps, invalid emails, invalid or closed-down accounts or domains, global DNE, hard bounces, soft bounces. Are you able to do all this? It is especially important to remove the spam traps. This is essential to maintain my good sender reputation.
If this project works out well, I would have similar projects very regularly for you.
What other services do you offer related to bulk emailing? For example, do you offer a mailing platform where I would have access to send myself?