That all depends on How fast you plan on growing and how much HDD space they are offering.
As for the Bandwidth if you are expecting people to stream the video you may need to look high i would also take a look at the budget vs income and plan a little for the future as if you are growing by 100gb a month you will very quickly out grow that server
It's a decent server to start with, it should be able to handle the 300 visitors.
What do the visitors do most of the time anyway?
You can always upgrade once you are growing, I'm sure your host can be flexible with that also
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I would personally want to make sure I had two drives in raid 1. Since you're planning for rather quickly needing 250gb of space within the next month or so, you'd be best off getting at least a 500gb drive. Providers may charge more for 500gb drives than they do for 250gb drives, but if you look at the cost to buy those drives, it's pretty much identical, so you should be able to negotiate to get a 500gb drive at close to the same price they're charging for a 250gb drive.
For redundancy and performance reasons, you'll want to get a raid 1 array of two drives if you can afford it. Read performance should be doubled, and if a drive fails, your server won't go offline because of it.
As far as the specs and price, yes, it's a good set of specs at a good price. The core2quad cpu isn't the best in the world anymore, but it's still a solid performer, and at the price quoted, a good deal as well.
I would also look at the growth options in that specific server. Honestly, I would go higher spec across the board and really deliver a solid performing server. Its far worse to have a badly performing server and have visitors tuning out- while you upgrade, than to grow into a higher machine and pay a little more upfront.