Looking for a way to include a one time download link of premium content with individual code for business cards that can be used as referral tools for clients. Any apps, companies, etc that help with this?
Check out Gumroad.com and PayWithATweet.com - those types of tools should give you a good starting point to have a capture method before delivering a digital good.
As for the couponing / code system you reference, I know you can do it with Shopify and some of their digital delivery plugins - but it will require a fair amount of time to setup (12 hours)
I believe you should invest some more time to simplify the idea. Content or business cards, either way it's going to be a database building and lead generation activity for you. The question is, what value are you trying to communicate? Is it the premium content or the referral?
The online marketplace out there is already undergoing the wrath of paralysis-analysis; all due to availability of too much diverse quality content. How would you ensure that yours is a premium one?
As far as including a download link is concerned, that's not a big challenge and could be easily done by any technology enthusiast. Is there something big that you're trying to accomplish? Feel free to reach out with more clarity.