Couple of points to consider:
1) Most of our clients require a CMS platform to manage the content of their site themselves which reduces ongoing costs. So ensure whoever you choose offers this.
2) ensure the site is modular - meaning as your business grows and becomes more sophisticated then so to can your site - without having to rebuild, you simply add in the extra module(s) like ecommerce or client access portals etc.
3) get exposure to se good, working and live websites that the agency has built. Like these by us:
4) the site needs to be responsive for mobile and tablet devises to make sure everyone gets a good experience.
Good luck. If you'd like to find out more about Catalyst you can go here: www.wearecatalyst.co.uk