If you are serious about riding then I highly recommend buying a brand name bike instead of budget bike that you buy from Pep Boys. You will spend a lot more for a Honda, Yamaha, Suzuki, Kawasaki, KTM, etc. but you will get a bike that will take the abuse you will give it and when something does break you can easily get parts for it.
It appears that the "Baja Motorsports" bike was a rather small 125cc four stroke. You definately need to be on a full size bike, minimum of 230cc four stroke or 125cc two stroke.
If you intend to be riding track a lot then I would recommend getting a full MX bike. Dirt bikes come in two basic catagories: MX (race) and "Off road" (trail). The race bikes leave out the frills, such as electric start and kickstands and will have higher performance engines. The trail bikes are built for more general use, may not have quite as good of suspension but are great for trail use.
In your price range I would recommend something like a Yamaha YZ-125. You shoudl be able to pick one up that is around 5 years old in your price range. This would be a great bike for the track and will serve you for a number of years.
Rod