No man, onza's are fantastic starter bikes, but at the height you are you'll probably feel more comfy on a stock (26" if you never knew). Or maybe a 24" which is a very streety bike, can be used as bmx trials if that makes sense! If you'd like a 24" go for the onza zoot, heard good things about that.
edit: And welcome to Trials-Forum mate!
Kieran.
No, some vee brakes are immense. Don't go double disc unless you can get it really cheap.