To be perfectly honest, $300 is nowhere near enough for a laptop/workstation capable of running Revit efficiently. My current Laptop (Dell Precision series) cost about €2800 a few years ago. I'm looking at upgrading to an Alienware in the next few weeks. I also use Lumion, and Lumion seems to work far better on GeForce video cards (as found in the Alienware machines) as opposed to the Quadro series cards (found in the Precision series). Revit doesn't seem to mind which card you use, unless you do a lot of rendering.
I also have a desktop PC which is custom built and cost around €1500.
Sorry to be the bearer of bad news but you may as well know the truth. No point in you wasting $300 on a laptop only to find out that it's not up to the task.
When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car.