I've been mulling over the idea of a new web directory with some enhanced features. The problem is that I don't know whether I should use a category killer domain name (exact keyword match) for a new site or 301 redirect it to an existing 12 year old site with a web directory section. Would it be best to build a new site with the new domain name or would there be any SEO advantage in redirecting it to the old site?
Based on your post, I would use the killer domain name - as the benefits of using that should outweigh the benefits of the domain authority of your 12 year old site. However it also depends on your plan/vision for the directory site. If you have reasonably big plans for it, I would definitely use the new domain. If you see it as more of an 'add-on' or relatively small directory/resource, then it might be worth looking at tacking it on to the existing website.
So, it depends on your goals I would say
Optimistically, it will dominate the Irish web directory market. But it would require some work to get it to that stage.
Well it is not going to be a global Googlekiller.
My understanding is that the main function of 301 is to preserve existing rankings when you transfer. As (I'm assuming) your new domain has no ranking a 301 redirect would be of no more benefit than, say, domain forwarding.
In the scenario you suggest I'd have thought redirecting the old domain and urls to the new keyword-rich domain to be the best option.
If there is no PR and it's a traffic grab - the new domain would be the better option IMO