Almost forgot...leave space in your budget for the accessories. It just makes sense to wear a helmet. I would strongly advise a couple of spare tubes, a patch kit, a multi-tool (preferably with a chain-tool on it), and a frame mounted pump. That will take care of any possible trail-side repairs. I use a wedge-pack under my seat to carry stuff in. A good bike lube to keep the drive-train running smoothly.
Look for information about local bike clubs. It's always more fun to have someone to share the ride with and riding partners challenge each other to get better. Plus, they can help you find good trails.
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