Business cards are a necessity. Many options exist from inexpensive preprinted templates available in stationary or printing service stores to more expensive cards designed and tailor-made by a graphic artist. Custom designed cards may run around $1,500 however, never underestimate the marketing benefit of a well-done, professional look. You want to create the impression with everything that you do, that you are a successful businessperson with a thriving business.
Stationary is helpful although not necessary. If you are starting on a shoestring budget, you can design a decent template with word processing software using the header/footer sections. You can then print your correspondence and invoices on good quality paper. Again, however, you may find that the additional credibility you gain with stationary is worth the effort and expense.
Marketing literature is important to leave behind after an initial sales call or to send in advance as a promotional/introductory vehicle. Marketing material will help distinguish you from your competitors. You should outline the services you provide, projects you've completed, names of satisfied clients, and a short bio outlining your skills and qualifications. Spend some time to define what is unique and valuable about what you bring to your customers. It may be specific industry knowledge you have, a specific technical skill set or a personal quality or skill that would make you easy to work with.
Professionally designed brochures will enhance your image. If you 'do-it-yourself', make sure you check your spelling and grammar carefully or have someone else do it for you. Nothing reduces credibility like careless errors.