Blogs
I recommend for your blog using the
Articles extra for blogging as opposed to having a whole separate site for such.
SEO
SEO should not be considered a plugin. There are basic things you can do to be sure your site gets noticed, but one thing you will always hear... content is king. There is far more that goes into such. Also, SEO changes on a regular basis, so unless the developer of any plugin is keeping up with the changes, you're likely to have issues where you shouldn't. SEO requires fine tuning for each site. The idea behind MODX is having the control over your output as you mentioned.
One thing I absolutely LOVE about it is the ease with which you create a theme
Title, description, and social media meta tags (you can forget keyword meta tags), making sure your textual hierarchy is in order (heading titles are consistent and your content is readable without keyword stuffing), and relevant content is a good starting point. With such, just use MODX's placeholders in your template or chunk (pagetitle, longtitle, desciption, etc), and you don't really need much else in the way of a plugin. If you have a client that needs to have easy-to-fill options, just use Form Customization to clean up the forms. This way you have control over each page, the options you want, and putting them in the right places. Don't rely on a third-party developer with a free plugin to stay on top of modern SEO or content strategy.
Social Media
Once again, make sure your meta tags (Facebook Open Graph and Twitter cards) are optimized for each page. Also, be sure to provide a thumbnail (with a TV), and set a default in case there isn't one used. As far as automatically posting, I recommend doing this by hand. I would go as far as using Facebook's Debugger to validate your URL to make sure everything is in working order. One way to speed things up, add the Facebook Link or Share and Tweet icons for yourself. This way you can customize in your meta tags exactly how you want it to read when they're shared. This also doubles as a way to debug or test it for end-users. Maybe an article uses tags... why not make those hashtags on Twitter along with any related @usernames? You may want a wide image on Facebook for some articles, but a small image for simple pages or categories. Give yourself the flexibility to control your social media campaigns rather than relying on something to work universally. It may save you a minute per article, but unless you're posting 100+ articles a day, it's not a major time saver. It can actually be a headache down the line, costing more time to fix (if there's a typo, an API is broken, or a wide array of possibilities.) That's what makes MODX powerful... it gives you more room to customize the final output without a bunch of hacks that are expected to work for every page the same way.