It’s good that you raise this question because people who are completely new to running a forum or a website will wonder about these things and it’s not easy to find an answer because much of the information online will assume that you already understand the following and its implications:
Discourse is 100 percent free and open source software. Using it to run your own forum will cost you nothing.
Running a public forum on 100 percent free and open source software will not be 100 percent free in practice because you need, as a minimum, a server with an internet connection to run it on. Although servers are incredibly cheap these days, they are not free. This is what you get from Digital Ocean for $5 and more. (See Recommended Hosting Providers for Self Hosters for similar options)
Apart from running the server (in the sense of turning it on and making sure it has Internet connection) you also need to maintain it (in the sense of updating the operating system and routing incoming traffic etc). This is what you get if from providers offering managed servers. For various reasons, managed servers are rarely used by admins of discourse forums.
Apart from a server to run discourse on, you also need a mail provider for transactional emails so that discourse can send emails to forum users. You need to have that set up before you can install discourse. You can not use an ordinary email account for this. Luckily, there are several providers that offer a free contingent of transactional emails that is large enough to run a small and medium sized forum an no cost (though free offers are rapidly declining). Basically, you can choose between Mailgun (10k free mails per month) and Mailjet (6000 free mails per month). Once your forum grows large and needs to send out more than 10,000 emails per month, this will be another cost factor, though not a huge one compared to what your server will cost by then.
There are still more “costs” involved in running a discourse forum. You need to install your free discourse software, keep it updated, install plugins, fix stuff that - in theory - should never go wrong, and so on. You can do that yourself or pay others to do it for you. www.literatecomputing.com offers help with these kinds of things if you decide to run your own server (e.g. at DigitalOcean). If you don’t want to be bothered with any setup, email services or server related stuff and want to start by logging into your shiny new discourse instance, that is when you opt for a discourse hoster like discourse.org or Communiteq (formerly DiscourseHosting).
For reasons of completeness, I might as well include the final level of costs involved in running your forum on 100 percent free and open source software: community management. If you are running your forum as the hobby of an individual, you’ll probably not consider this as a cost at all, but if you are an organization, this is likely to be your largest cost-factor in running your forum: you need to pay someone to manage the forum (or the community) on a day to day basis. Answering members questions, tweaking settings to suit the needs of the community, handle spam and otherwise problematic posts etc. In a well functioning community much of this work can be taken over by the community itself (and discourse is built to facilitate this) but some admin work will always remain.
This was a bit more information than directly warranted by your OP (and I assume that much of it was not new to you), but I thought writing it up like this might be helpful to others who are trying to wirk their way into running a forum.
So, the short answer is: when you’re comparing discourse-hosting providers like discourse.org or Communiteq (formerly DiscourseHosting) with server hosting providers like DigitalOcean, you are really comparing apples with oranges. (And there is, of course, nothing wrong with comparing apples and oranges as long as you know that that’s what you’re doing.)
What is still confusing me , is which of these options lets me install the plug in I want - topic list previews. I assume digital Ocean does. But the other 2 I’m not clear on.
Also the mail server aspects really confuse me.
Not talking about reply by email here. But I assume there’s always a need to set up a mail server to send the emails sent out by discourse?
Is it always an additional cost and set up?
or Do the managed services include the mail server in their monthly cost?
Try monetising your forum and you may discover it’s hard to raise >$40 a month. Unfortunately in this day and age people expect these communities to be free.
By minimising cost you can make it a much more sustainable concern putting less pressure on your revenues.
This all depends how busy the forum is, who your audience is and the subject matter of course. You may also find unique ways to monetise due to specifics of your community.
Our plans at Communiteq (formerly DiscourseHosting) include outgoing mailservers with a good reputation, as well as a POP3 mailbox for incoming mail, for no extra cost.
On the Professional plan and up, we can also configure your own or a third party mail service if you really want to.
We will install the topic list previews plugin on the Professional plan and up. Setup is free and you get 5 plugins of your choice next to the standard plugins (Akismet, Slack, Solved, Adsense, Patreon, Sitemap, Data Explorer and Staff Notes).
Your first step here should probably be to fire up a $5 droplet at DigitalOcean and give the install a shot. You will be able to run multiple plugins and make mistakes with little to no risk. Pretty quickly you’re going to learn if self-hosting is for you, and if not have some idea of how much you’re willing to pay others to do it.
That $5 is prorated, so if you decide after 24hrs the exercise was a mistake you’re going to be out of pocket to the tune of… $0.16
Mailgun will be free below 10k emails/month as long as you don’t do anything daft, it’s very low-risk and will help you learn about the platform as you go.
Digital Ocean is great for taking a test run, but get into trouble self managing this and all the time you’ll invest to get it tuned to your needs and $100 per month starts sounding really good. I had an instance go bad and the backup wasn’t even working to restore.
Digital ocean has been working for us - Low startup and has a plan that includes Discourse along with scale-ability as resources are needed. We use their WAF and backup instances.
I can usually help people out for $100-300 when they get in the weeds with self-hosting. Self-hosting also allows you to use plugins that would require moving to discourse.org’s Enterprise plan. I have done installs for several hundred self-hosters and though I do command-line upgrades for some of them, a whole bunch of them just keep going for more than a year before they need my help again.