I wouldn't buy SEO from a general SEO company. Full stop. Marketing, maybe. Reputation management, maybe. But not SEO.
If my designer can't make a Google-friendly website then he'll be sacked as a designer. I don't need an SEOist to do my on-page SEO, because anyone half-way competent will be doing that all for themselves as a matter of course.
And I don't want to pay someone to go around blogs and forums spamming them with links to my website, which is what a lot of SEO companies seem to do. Unless you are a specialist in my industry, you won't have the first idea about which websites are going to be appropriate and effective to approach for links, how to go about it, what words and phrases searchers are likely to use, who my competitors are, etc. Hiring a non-specialist to do work that needs industry specialist knowledge seems like a pretty odd way of going about things to me!
Sorry if that isn't the answer you wanted to hear, but I can't think of any circumstances that I would advise anyone to buy in SEO services from an SEO company.