Blog content and links from blogs are supposed to be considered more natural, and therefore given better weight when ranking a particular page or link.
However, a blog with little or poor content is useless for SEO purposes. Paying £600 for a blog will get it built, but it won't do anything to improve ranking. Working on your blog and frequently submitting fresh content will. In addition, with good content, it's likely to improve repeat-traffic and therefore boost the usage and popularity of the parent site. It won't directly improve the main site, but it will drive targeted traffic towards it, and that should really be the aim.
The design and functionality of blogs is of very little importance outside the world of web designers who use it as a showcase. The content, like all the web, is the most important thing. I'd recommend setting up a basic blog yourself, and then spend the cash on getting a good copywriter to author some relevant articles to start you on the way.