I guess it depends what site you are guest blogging for. I don't think a guest blog on Forbes or Entrpreneur.com would be viewed as spammy in Google's eyes IMO.
Everyone above has nailed it.. On-page SEO is just as important as off-page SEO. It is best to make sure your on-page SEO is in check and following best practices.
Add a rel canonical link to your website to let the search engines know where the original content came from. If you simply place a URL to the source, you are risking facing a duplicate content penalty via the hands of Google...
It depends on a lot of factors.. What are you trying to accomplish? What I can define as my top 10 go to plugins, you may have no use for.. Are you looking for SEO plugins, Social plugins.. What?
If you are looking to monetize a blog, definitely go for Wordpress. I have used workpress to build all of my sites, and I could not be happier. WP is super customizable and there are tons of plugins that are at your disposal.
As stated above Odesk is a great place to source content writers from. You can set your budget and the parameters for the work you require and choose a freelancer that meets your needs. I tend to use eLance more often than Odesk.
Focus on a long-term white hat SEO strategy. Create a plan to build SEO value to your website over time. If you do things correctly, and adhere to Google's SEO best practices, the rankings will follow!
It really depends on what you are trying to accomplish. The best thing about wordpress is that there are different options to suit different needs. Wordpress is not a one-size fits all sort of CMS.
Guest blogging is a great way to build SEO value to your website. You establish yourself as an authority figure in whatever niche you are contributing to and you are able to build valuable backlinks that will help your SEO rankings..