I like to joke that this is a business that funnels money from customers to Google Adwords, while I take a small cut for actually delivering the product. But even if that turns out to be the case, it's not so bad. I have few costs other than support staff and marketing.
I'm sure you can get decent traction from adwords in this market, but I would think that you could do wonders with seo here. The number of blogs you could write and forums you could comment on have got to be nearly endless.
I think it's a good idea to do both. It can take a few months to see results from an SEO campaign, so in the meantime, I'll do Adwords to optimize my landing pages and figure out which keywords convert the best.
Have to agree on that good Seo is the key. I set up a while a website for my mum allowing her to offer some online service and there is a tremendous demand. I just wish the service was as automatizeable as yours. Good luck with that, its a great idea!