I know your immediate questions are "What version of Moodle shall I install now?" and "How big a server do I need now?" However, in the future you will need to answer the questions "Can I easily upgrade to the next version of Moodle after it has been released?" "Are my current servers performing OK for the ways my users are interacting with Moodle?" and "How can I move my Moodle site to more powerful hardware?"
The thing is, if you can answer those future questions, then you do not have to worry too much about the current questions, because you will be able to adapt later if you need to change things.
To answer your questions:
If you are starting now, the choices are to use Moodle 2.8, or wait a few weeks, and consider starting with Moodle 2.9.
For a Moodle site of your size, it is not just a matter of the number of users. It makes a big difference what they will be doing. If they are just downloading a few documents, then you don't need much power, but if they will be doing lots of online quizzes you need a more powerful server. Putting everything on one server will be faster for a small site (no network connection between Moodle and the database) but it will be harder to grow in future. I am not really an expert here.
To answer the long-term questions:
How to upgrade? Well, the basic process is here: https://docs.moodle.org/28/en/Upgrading. A key point though, is that it is really good if you can test the upgrade on a copy of your site first, before you do it for real. It is also important to be able to make a copy of your site because of disaster recovery. You need to know that you can restore your backups onto another server, and get your Moodle site running again, and that is almost the same process as making a copy to test the upgrade to a new version. See https://docs.moodle.org/28/en/Moodle_migration.
How do you know if your site is performing OK? Well you need some basic form of monitoring. That could be as simple as analysing the server logs from time to time, to see how long pages are taking to load. Or it could involve setting up a tool like Nagios to monitor the servers in real time. Your ISP might provide you with tools. It is also important to look at things like CPU and memory use on the servers. See https://docs.moodle.org/28/en/Performance_recommendations for tuning advice.
How do you move your site to more powerful hardwhere, when you need to? Well, that is https://docs.moodle.org/28/en/Moodle_migration again.