Hi
What is the minimum that I need in terms of marketing materials to approach businesses and offer my services?
I would imagine a business card is an absolute must.
what else? Do I need brochures? What about those papers that show your portfolio?

how should my business card look? I will be ordering prints soon. I found the picture below on some website. is it good? what should the back side say? (I can get the back for the same price)

I have cards but I don't use them a whole lot unless I'm meeting face to face. I think it depends a lot on how your marketing to your clients. Something that most clients want is proof. So if you make a brochure to hand out put some case studies.