When searching for a web developer, it is important to identify two things:- (i) does the developer use custom-made designs or (ii) does the developer use templates? Both have pros and cons; the former offers you complete control on how your website will look and function but will be costly and the latter offers you limited control but more economical.
Second biggest consideration is how complex a website are you seeking to build? If you want a full-scale eCommerce website with 5 currencies and 10 languages then not many of the current open source platforms would be able to support your need and you will likely be better off with a ground-up build using frameworks or deploying a proprietary system that has many of the functions you are seeking.
Unfortunately, in the web development space there are many new comers who promise the world and have little experience in web development beyond downloading and deploying wordpress templates and plugins. If you come across an agency who has less than 5 years experience you have to be slightly cautious; do the due diligence and talk to some of their existing customers to see what their service is like.
Lastly, when developing a website; make sure that you test across all browsers (Firefox, Chrome, Internet Explorer, Opera, etc.), Different screen sizes and devices, Loading speeds and make sure that people can find your website on Google, Yahoo and even Bing. If people can’t find your website then it’s not much use.