hi,
I have been doing motor trials for about 2 - 3 years now and we need to get one thing straight.
Lots of people (the cool kids) think that having a higher cc engine is better and so much cooler.
Well it isn't (maybe in moto-x) trials is not about speed, it is about balance, skill, no-fear and the passion of being a trial rider.
You should go for a 125cc only because it is not too powerful and it is alot lighter.
Gas Gas tend to need fixing alot, but they are a good bike.
Do not spend too much money on your first bike, when you first start you should not care what your bike looks like until you get better.
Go on Youtube and watch videos of the top riders:
Dougie Lampkin
Toni Bou
Adam Raga
Albert Cabestany
Alexz Wigg
James Fry
All of those riders are really good and there are more out there.
So if you need anymore help then just say so