More information on what you intend to use it for would be helpful: "park sand/dirt trails" can cover everything from groomed limestone to rough, through the woods single track...
Also, more information on your riding back ground. How often and how far do you usually ride (or intend to ride?)
At 5'4" you will need a smaller bike. But, at least from bike shops, they are readily available. it may be difficult to find used bikes in the smaller size -- but you never know...
The pure mountain bikes with knobby tires are for riding undeveloped rough trails. Hybrids are essentially a cross between road oriented bikes and mountain bikes and come in a wide variety of orientations: For example (not as a suggestion) Trek produces the FX line (which is a road-oriented hybrid) as well as the DS line (which is oriented more to the mountain bike side)... I think the base models in each line start in the $500-$600 price range -- which is well above your goal.
You have come to a good place to start: people on this forum have a wide variety of tastes and experiences.
I think most will tell you that you need to narrow down the type of riding you intend to use the bike for -- and that the fit of the bike is perhaps the most important criteria. And, the best approach usually starts with a visit to a local bike shop (not a big box store). A local bike shop has addressed questions like yours quite a number of times and will be equipped to show you a number of bikes that will work well for you -- and even let you test ride them....
In the end only you will know what is best for you. But this forum and your local bike shop are a good start.