1 - Again- answering this for the same client company with which I've been using Hubspot for 3 years. And I'm not in-house so I guess the correct answer is zero. Not counting the COO (who is hands-on in terms of ROI and poking around at performance metrics, etc.), we have had what's called an #inboundteamof1, which is me. (There is actually a group on Twitter and G+ devoted to this, on account of its particular challenges).
You don't need a marketing expert, because Hubspot really will provide the necessary education. However, when you think about the often-touted 5-10 hours minimum commitment, you have to factor in an hour or two (per week, when your starting) for education, and then if you want to get one content offer up, well that's one thing but aside from the offer itself, you also need to have a call to action, a landing page, a thank you page, a follow up email, so depending on what it is (and your quality standards), it's not unreasonable to think that might take 5-10 hours for one new offer. Steps to post a single blog can be similarly deceptive, not on account of Hubspot but on account of quality (I think 2-4 hours is generally realistic/typical including all editing, formatting, graphics, meta, depending on depth of topic. Some say 1 hour; I say those are exceptions or they suck...you can't post list posts or 1 paragraph posts every time and expect positive results).
When factoring in how many people you need, do you want two blogs posted per week? Do you want 1-2 unique content offers/calls to action? Do the math. Want someone who can take all of Hubspot's great performance metrics and apply them to actionable priorities? Well, that takes some time too. Not much, but that can easily be an hour a week or a few hours a month, depending on how much you want to get into it. Emails? Social Media? Workflows (a GREAT asset, btw!)? Time, more time, and more time. By the time you get everything in place, it will be time to go back and revise and update what you have, replace non-performing and dated content with better content, and so on. It is easily a full-time job for one awesome person working her butt off- if you can afford to do so. Or maybe 2-3 people working part-time, if that makes more sense.
Back to Hubspot's education- it extends beyond Hubspot's product and is also about the inseparable topic of inbound marketing best practices, much of which would benefit you even if you left Hubspot. Whether you see this as a chore or added value for your organization may tell you a lot about whether Hubspot is right for you. I'm not sure how much less time it would take if you start with someone who is a marketing expert but not versed in Hubspot...less (but how much less?)....if time is your concern and you are bringing on someone external, be sure to get someone who knows Hubspot well.
There's no way to say with certainty how much time (or people) all organizations would need; it depends on your goals, scope of services, target personas, geographical target, how big your site is ....etc. If you or your team is enthusiastic about maximizing your online presence, you may find Hubspot makes a lot of sense for you. If you are open to integrating it with your sales efforts, rather than just tacking on 5 hours for someone external (in every sense of the word) to "do Hubspot", then when the inevitable ROI question comes up, you will not only know exactly where you stand, but you will likely see much better results. That's not people/skills fit as much as organizational.
For your content, you would need to assume at least one person on your team has above-average editorial skills. Generally, I would say at least one very smart and competent person with advanced writing skills who is tech-friendly more than phobic, someone who loves to learn, and whatever other marketing, SEO, website, design, or editing skills they bring are a plus. And, if they are writing about your business and to your target audience, they also should understand it. If you are hiring an outside consultant for 5 hours a week, think about how that would happen. 5 hours goes quickly.