Is there much of a difference between flatland and park/street frames?
Since you are new to BMX I would find a good local shop that specializes in BMX. In my area the three largest conventional road/MTB shops don't carry BMX. We do have two very good shops that only sell BMX and skateboards. So don't be dismayed if the first shop you check has nothing you want. If you can't find one, stop by the local skate/bike park and see what everyone else that is riding. Ask where they go for service, repairs or parts. Like mountain biking, BMX has bikes specialized for specific styles. The frames and components are designed for the style. From what I have seen Flatland bike are designed for slow speed tricks on flat terrain. This is where handling and light weight are more important than the ability to survive big air. The info I can find seems to point to you able to do flatland tricks on a standard BMX frame. If you try to do some dirtjumping or big air tricks on a flatland frame it's lightweight frame and components wont take the abuse for long. Since you are new, your style and preferences will change. I would look for a good used bike, this is where a good local shop can help.