User profiles, dashboard, and capabilities

I’ve spent a lot of time looking around this site, and though I’ve been learning things I admit I’m still somewhat overwhelmed. I have some questions for anyone who wants to take the time.

My major concern right now is making sure I have my site set up so that it’s simple for users and they only get access to certain features.

The lowest level is a free account. Users will only be able to post comments, take part in polls, basic stuff. I see no reason for them to get access to the dashboard. Aside from the login/logout redirect plugins, is there a good way to go about denying access short of blocking ip addresses? Or should I not worry about the possibility of them coming across it on their own?

The next level should allow simple blogs/sites. I’m thinking about using the Quick Post widget, though I have yet to test it. Again, I don’t think the dashboard is necessary but I can live with users getting access.

Now for the highest level I do want to keep the dashboard as I want to give members more options and flexibility with their sites. My concern here is if anyone has any thoughts on allowing the use of different themes other than the main one considering that I’m taking the sub directory route. If I was using sub domains I’d have no problem with it, but I’m just not so sure here. I’m trying to look at it from the perspective of visitors who are probably accustomed to a certain level of consistency within websites.

For account upgrades, I’m not so sure how I would go about making the page available only to logged in members? Or any page for that matter? Is there a way that the sign up page could be set up to automatically convert its options to upgrade/downgrade if it detects a user is logged in?

And finally how would I go about setting up public profiles? Maybe I’m not looking in the right places as I haven’t found much on this.