I have been using Siteground as well for almost 1 year. I currently have 5 course and 200 students enrolled. I have one course where I have 47 students connected at the same time and it is a little slow but very usable, usually when they are downloading video or audio files is my biggest bottleneck. As for backups, yes once a week isn't ideal, but most of my classes here in Japan are held once a week, so for me it is fine.
Yes, they are using an older version of Moodle, but unless you really need the latest and greatest 1.5.2 is really great. I have my own server at home I use for 1.6 as a test as well as for 1 class of 15 students.
The cost vs features was my main decision to go with Siteground, I have to pay out of my own pocket. If you have the money I recommend going with a Moodle Partner in your area as they will be the most knowledgeable and understanding of your needs.
I am sure some people will say that Siteground is awful, but you can't please everyone.
There are 16 active courses and ~85 students on the server and it runs fine (without chat mind you) but I see that if the traffic picks up and the number of students increase I'll have to buy new hardware.
Your demands are low. Any recent computer, say Pentium III upwards with 512 MB RAM can handle that. Note the amount of RAM, which is more important than the raw processor power.
People use Bandwidth to mean many things: in hosting some mean the capacity of the upload connection to a backbone, measured in MByte/s oder GByte/s. Others mean, volume of data transferred during a month, measures in GByte or TByte.
Those things you can caluculate youself. How much download material, how many students, how many re-downloads, etc.
Hosting space the same. The whole Moodle collection might need 50-100 MB, everything else is _your_ material. The volume of the MySQL database itself is small.
I am a newbe, or should I say a newmoodlebe. It appears an external host can be used, your own server, or a Pentium III or better with a minimun of 512 ram and a 50 to 100 hard drive.
Can you avoid above and use just one machine with sufficient ram and hard drive memory? I am using my own resources in exploring Moodle for a business training application.If so, what suggestions might you share?