Building a beginner trial bike
Perhaps you'll have more luck with answers on specialised trials forums, like http://www.trials-forum.co.uk/. However, I'll try to address your questions briefly. 


Risk to damage the fork/frame? Yes, absolutely. But all depend on what you are doing and what bike it is. I've snapped a few frames doing trials and stunts. But chances of damaging are less if you are using trials frame.
Wheel size is a great debate in a trials world. There are 26", 24" and 20" (19") wheels. All down to the preference and rider level. I'd start with 26" and then once you know what you want/need, decide on the sizes. If you are very keen on trials and have finance, definitely get a trials bike. Many times while learning I found my bike stopping my progress. 
Good quality parts... get a good chain and good freewheel. Because if freewheel slips while doing gaps - you are going down. And that can lead to an injury. Also get a great rear tyre with good brake. A lot depends on that. Other parts are essential, but these are high priority.
Breaks are another point of debate in trials world. Magura HS33 - hydraulic rim brakes used to be very popular 10 years ago. Now I'm not quite sure about the trends. Certainly I'd put some cash in for a good rear break - that is a vital part of the bike.


A lot of your questions are down to experience. You'll know when you need a better rear break and what other parts to change. And you'll slowly assemble a great bike, going along with your skill level. 

And good luck!