Depending on the type of sites and the type of content, 300 sites can bring a server to a standstill or it can zing along without any load. It is only a partial indicator of what is going on. A really good indicator however can be disk loading and cpu usage. Cpanel hosts "can" make this information available to all the users. Typically an over sold server will run from 85 to 98% disk usage and CPU loads will run at similar levels. But the fact is you typically get what you pay for. Without getting to specific a server needs to generate $800 per month to be viable for the owner. You can do this with 50 sites or 500, and it does matter to the users which business model you use.
Many (and I mean a lot of hosts) will go way out of the way for clients. Where this does not happen is for some of the largest of company's. They really have no need to cater to a small faction of users such as this. In the years I have been at this, I have found "new" users are the normal trouble ticket and after about 30 days, we do not hear from them again for a year or more.
We as well as a lot of smaller company's (less than 2500 users) do a lot of custom work for clients (we had to learn all about Zope a couple years back and it like Moodle, has a very defined group of users. Very often a user has a coding error and needs help fixing it. As faulty code can genuinly disrupt a server, it is to our advantage to help them get it fixed.
It does pay off to use a host that others can vouch for after at least a year of service. As I posted earlier, just ask them for what you need and see what happens. Having attended a number of symposiums and seminars for the hosting industry, I can say without a doubt that there are hundreds of capable and willing companies that will bend over backwards to help a single user.
I agree about Fantastico lagging behind a bit in versions but the guys at Netenburg do a pretty decent job of keeping current and secure versions up to date. I personally have no idea the differences between the versions of Moodle, I only got involved with this because the Community College here needed it installed and they could not do it in-house.
Moodle install is pretty much a no-brainer anyway, the Fantastico however allows "Anyone" to do the install. It may be possible to update the Fantastico version to 1.6 and in the next day or 2 I will see about this.