What I think , when you are planing to do SEO of your company is that:

Before looking for the right SEO company ,you should do your homework,study in detail about SEO.

Try to figure out the common bugs in your site.

Try doing your SEO in house..it proves to be cheaper.

But if you really want an outsider to do it for you because you cant handle it...then go with someone certified and a proven track record.

Track the progress of your site in early months...because if the seo is done good, you will start noticing the chunks of progress in early months.

And yes , I do agree with the fact  that most of the SEO providers do not focus on your onpage SEO that much ,but on page seo is equally important as off page SEO.

Remember, quality of the content at your site plays a major role in boosting your business.Focus on the graphics, content and other on page factors.

This means , you need to focus on on page and off page both equally.