Both are very important. Find a designer that has mastered a good balance of presentation design and functionality design. Ask for a portfolio of previous projects. This will give you ideas on how sophisticated a Web designer is and also give you a glimpse of his or her problem-solving design style. You can ask your Web designer for a quick budget estimate on what it will take to create and maintain your Web site annually.
Web sites are very dynamic.
You want to be able to update your site without it costing you anything. A good Web site should be easy enough to update under your control. You don't need a cutting-edge technology on a Web site. Usually, it just slows the site down. Keep it very simple.