Don’t expect to hit 50 kph, 30 kph is probably within your grasp though.
-What things make the 36" go faster?
Gears! (I’m assuming you mean, what will make a 36" go faster than a standard 36"). A Schlumpf hub allows you to gear the wheel size up to 1.5 times normal, giving you an effective wheel diameter of 54". Larger diameter wheels go faster as each turn of the pedal takes you further. Crank length is a factor too. In general the shorter the cranks, the faster you can spin the wheel but the harder it is to climb hills and control yourself.
-Any advantages in getting a more expensive tyre like a “Night-Rider”?
It is slightly lighter, which means less rotating weight, which means it is easier to go faster. The tread pattern is also better suited to offroad riding if you want to have a go at that as well as road riding.
-What crank length is standard? Which goes fastest? When do I know what size cranks to use?
Crank length is a personal preference thing. Riders just getting the hang of 36" wheels usually start out with 150mm cranks and then use shorter cranks once they get used to the wheel size. I use 125mm cranks and some of my faster friends use 114s.
-Can you get gears on a Coker? Does a 29" GUni go faster?
Yes, you can build up a KH/Schlumpf 36" wheelset and pop it into a KH 36" frame. You’ll need money to burn though. Those things don’t come cheap. (eg probably around the $3000 AU mark)
-Will my legs even reach the pedals on the Coker? I’m 13 years old…
Depends on your leg length. You’ll need a minimum leg length of 75cm to ride a standard 36" uni without modifying it. Leg length is measured from crotch to the floor with shoes on, not your trouser length. Some shorter riders cut down the frame so they can fit it.
-Why get a Schlumph hub?
To go faster. They are also uber cool.
-Why get a Double-Crown?
If you mean a Nimbus 36 frame, they are less likely to bash your legs on. (Not everyone has problems with a standard frame but I certainly prefer the Nimbus). I like the look of them too.
-Why get strong cranks if your just cruising on the road?
In case you want to go offroad 
-Why have brakes (Cant you just slow down but not pushing on the pedals?)
Certainly you can, but slowing a big wheel down takes a lot of muscle power and energy as well. For long distance touring you may want to avoid working your legs too much on the downhills. Try riding 100km over a hilly course and you’ll discover why it would be nice to have a brake. They are handy for allowing you to quickly descend steep hills without losing control too.
-Why get handle bars?
Some people find them more comfortable for distance riding.
-Do you think it’s silly for me to get a Coker?
Not if you want one 