Internal vs External (or new domain) for supplemental blog

I've searched the archives and found questions similar to mine, but different enough that I'm still a bit lost. I apologize in advance for the long question.

We run a relatively high-traffic non-profit web site with brand-related keywords for which we occupy a multi-link top spot for natural search results in Google. We are wanting to add a blog as part of our efforts to expand our web presence. The goals of the blog are:

1. To give a new voice to efforts in our organization which are well-suited to a blog format

2. To increase search engine visibility for these efforts, which currently have content within our web site but which do not appear in natural search results for pages or pages

The blog would contain content from several efforts, would probably see a burst of activity up front as we stocked it with content, and then would receive new written content maybe once per week. Content (articles, pages) would sometimes link to external sites working in conjunction with us, but would always contain links to related content on our web site. (The blog would be updates and discussion about the effort, and the link would be to static pages on our main web site that actually describe the efforts in detail.)

While we do want to improve the natural search results for these efforts, we're obviously not really concerned about our main site's natural search placement. (We're not fighting it out with anyone and likely never will be for the top spot in any of our key brand-related searches.) We would just like to develop more traffic for these efforts specifically.

We would like to use Wordpress for our blog. We had originally planned to use Wordpress.com itself, but now believe we should host the blog ourselves using the Wordpress software. Here is where our questions come in, arising from:

1. We cannot host the blog in a subdirectory of our site, which is hosted on a proprietary platform.(wwwurcompany.com/blog) There is no possibility of changing this setup.

2. We can create a subdomain to host the blog (blog.ourcompany.com) but the blog will have a completely separate IP address as it will be hosted on an entirely different set of servers locally. (The main web site is hosted externally.)

3. We also have the option of simply setting up our blog on a related but separate domain name entirely. We had originally thought that this might actually help get "our" content seen better, even though the traffic would not be going directly to our web site. We can easily integrate a new domain like this into our marketing.

Given what I've described, does anyone have any advice on the preferred option here? We don't want to run afoul of standards, but we also want to be sure we don't set something up that ends up being ignored or even hurting us. Any advice is appreciated, or pointers to online resources that could help with this. I've read through the archives here until my eyes bleed, and scoured many other web sites, but the information is sometimes contradictory, and nothing really touches on this specifically given our situation.

Thank you.

My blog is in a folder on my site.

If I put it on another domain it would not enjoy the authority, trust and links that my own domain enjoys. That means my blog posts rank a lot higher because they are on the same site.

It is on my own site because I want no mistake about who that blog belongs to.

It is not in a subdomain because if I have a blog post and an article on the site about the same topic (this always happens because new articles are announced in the blog) we often get #1 and #2 indented listings in Google for the topic.

It is on the main site because it attracts links and the power of those links will benefit the site.

I do have another off site blog for topics that would stink up my site.