The kind of content you create depends on your audience's needs at different stages in the buyer's journey. You should start by creating buyer personas (use these free templates, or try makemypersona.com) to identify what your audience's goals and challenges are in relation to your business. On a basic level, your online content should aim to help them meet these goals, and overcome their challenges.
If you're focusing on inbound techniques like SEO, social media, and content creation for a preexisting website, the good news is you don't need very much budget at all. With inbound marketing, the main focus is on creating high quality content that your audience will want to consume, which unless you're planning to outsource the work, the only investment you'll need is your time.
The role of a social media manager is easy to infer from the title, but which social networks they manage for the company depends on the industry. Above all, social media managers establish a posting schedule for the company's written and visual content. This employee might also work with the content marketing specialist to develop a strategy for which content to post on which social network.
The digital marketer usually focuses on a different key performance indicator (KPI) for each channel so they can properly measure the company's performance across each one. A digital marketer who's in charge of SEO, for example, measures their website's "organic traffic" -- of that traffic coming from website visitors who found a page of the business's website via a Google search.