What material should I use for a platform that holds 300-500 lbs?
300-500 lbs is not really THAT much, spread over 4 square feet. That's the weight of about 2 people. Your question is a little vague since you don't say whether this is for indoors or outdoors, whether this is a raised wood platform like a deck, or a concrete pad, or what.

If you want to use wood, you could definitely just use 2x's like for a deck. If it's not that high, you could put it on 4x4 posts and that would be plenty strong enough. You could attach your leveling feet under the posts.

To reduce vibration you could use rubber, although whatever you do you should make sure that the fan is properly secured. You can get round rubber cups that go under washing machines to reduce vibrations: that might be one way to go.