With all due respect, I don't think that you've really researched the SEO market very well, based on your concerns.
I have couple issues.
1. I don't have time to do SEO myself.
2. I don't have time to optimize 300 keywords and managing PPC and be able to get all these keywords on top of google.
You'll never get any given set of 300 keywords to the "top" of Google, particularly if they are generally related terms. On the off chance that you were not using hyperbole, just for the record any firm which claims that it can do so -- for any price -- is simply lying, and will get you in the fine print.
3. We are looking for hundred of thousands or million visitors per month.
While I believe myself or someone outsourced can do SEO, it's hard to compete with competitors on the first page who spent hundreds of thousands on SEO.
4. Random outsourced service or random freelancer seems to have the tendency of using Black Hat SEO. It might work for short-term like 1-2 years. Who knows we will get banned from google down the road.
I don't know how a specific service provider referral from an established member at both sites involved qualifies as "random outsource/freelancer" but setting that aside, any SEO firm you retain is just going to subcontract out all the grunt work to the same people, and charge you a 200-300% premium for being the middleman.
If you'd check out the other site I recommended, seobook.com, Aaron Wall does provide SEO consultation for $300 a month, and he is generally considered one of the most knowledgeable people in that field. You might also have a look at seomoz.org; it is a phenomally useful software and service for building and tracking SEO efforts. Its clients include Facebook, Disney, Yahoo and a number of other very big names, and it costs as little as $100 a month. The "elegant storefront" SEO companies are simply taking the information they get from places like SEObook and SEOmoz, and the services they get off of places like WickedFire, rating them up by hundreds of percent and popping a nice sales pitch on the front end.
Sincerely trying to save you what could be a significant amount of money. If you're looking at such an ambitious project, and are prepared to shell out funds for a consultant, you'd likely come out far better just hiring an office person at secretarial rates to manage the campaigns you generate out of data from sites such as those I've mentioned. The amount of cash saved is significant enough that it's worth considering.
"Failure is not when you fall down. Failure is when you don't get back up."