Since your primary purpose is to make sales through online shopping, you should look for a firm that specializes in that area for starters.
After you find some firms within your budget, ask questions, ask for references, and ask for previous experience and results in your niche.
Next you should ask details about how they build links, what types of sites related to your business they have gotten links from and so on.
In terms of devising a strategy you should focus on quality. Let's say you can afford $500/mo for SEO. If it was me, I would devote $300 to content creation for sharing on guest posts, social, and so on. I would spend $100 on fresh content for a blog attached to your site to generate interest and attract links. I would spend the last $100 just doing good old fashioned outreach to other authority sites in related fields.
The content you created has the ability to build an infinite amount of links at no additional cost to you. The blog will make your site easier to link to since it is not a direct sales page, and the outreach will give you editorial links that rock.
So to answer your percentage it will be 80% off page SEO and 20% on page. This is after you complete the initial on page SEO.
Good luck on your search.
-Travis