1) the hinges are your priority. I have never heard of "disk" hinges, and I am not sure about their reliability. Personally, I would replace them with some CA hinges. Like heath said, some don't like them, but I have used them successfully in all of my models.

2)again...up to you. I have only used it on one model and have never had a fuel/exhaust seepage in all the rest.

3)Definitely fuel proof the firewall. In my first trainer, the instructions also said nothing about fuel proofing, and thankfully, I had an experienced modeler to help me look it over and fuel proof it before it was too late. Epoxy or fuel proof paint will work just fine.