Hire SEO eCommerce Marketing Company, or Train my own employees?

We launched our large eCommerce website and a couple niche websites and we are getting good CTR's on a good amount of items and it seems logical to spend more on climbing in the search engines and getting more of our items viewed and eventually work towards building a brand. Our inventory at this consists of about 500,000 items, and we would like to ramp this up to 10 million by this time next year which would be easy and not really time consuming because we are essentially "dropshipping" but with a much more hands on method.

The problem is, when I have spoken to a couple companies in the U.S., we are told they can't handle that kind of volume of inventory, they can't work with rotating inventory of that quantity, or they want a 7 figure budget each year to essentially handle a million dollar google adwords budget but will do other forms of advertising they do not go into much detail about. I see that kind of investment would be worth it, but it just doesn't fit our current budget, and when we say "We have full capabilities to launch 10 million items online at very competitive prices" dollar signs start spinning in their head. Just finding a company that deals specifically in eCommerce SEO is more difficult than it seems with all of the ads I've seen online.

Should I continue to hunt for a marketing company that meets our needs, or should I look more towards hiring a consultant or two, and getting them to train 4 or more Chinese employees at the rate of $1200/mo USD. 4 at this time is just a random number, for all we know, we might need 20 employees.