Downhill biking is for people that have been into cycling for 10+ years.
I've been cycling all my life and a single downhill ride scared the sh** out of me.
First. Downhill isn't about riding down the mountain, it's about surviving the trail. You would need about $1500 for a good downhill bike and any lower would risk your life.
Second. You don't have enough experience for Downhill. Just try to go 35mph down a flat road, it will scare you, now think about if your flying down a mountain at the same speed.
I would say get a Specialized Hardrock Sport $400, and do some easier mountain trials. You can get them at a lot of bike stores, I like the Hardrock as a beginner bike because it has good parts and a great frame.
And highly stress not to do Downhill until you have 5000 miles under your belt. YOU WILL DIE IF YOU JUMP INTO IT WITHOUT PROPER TRAINING