Make sure it doesn't have u-brake "990" mounts.... Most street/park/jumping 20"s have u-brakes (or no brakes at all).
pay special attention to the last paragraph of this. the only bmx frames you'll get with v brake mounts these days are race frames, which are designed for racing and not messing around on (although some can handle trails and a bit of light street/park). half decent forks with v brake mounts will be very very hard to come across. you'll probably need custom built frame and forks, especially forks.
i'd like to point out though, hydraulic on a bmx is just a bad idea. you're much more likely to be throwing your bike about and accidentally dropping it all over the place, than any other kind of bike. Magura brakes are probably what you'd be after, which are easy to bleed and maintain (for the most part), but even still, will probably need fiddling around with every time you ride.