$300 is a really difficult price point for what youre asking for new. At the price, youre in the chromebook range, where even the windows machines are going to be as barebones as possible.
You want to step into the used market if you want customizable for $300. Getting something good thats a few years old like an lenovo carbon x1 looks possible, and they are a dream to update. The above supports linux with no issues.