Try finding the 'Training FAQ' thread in the Rotorheads Forum.
All of your answers will be in there.
But in short:
What happens if you fail an exam, can you just retake it?
Yes. But your number of retakes might be limited.
If/when I pass I plan to do my IR. If/when I have that can I get a job 150-200H?
Onshore: no. Offshore: maybe. (But at your age, especially once you've finished training: probably not).
What sort of money would I expect to earn?
IF, and it's a very big if, you did get a job as a NS FO: low £30s
Can I build up my hours doing navigations or something in a fixed wing if I trained in rotary?
No. They are different licenses.
These are obviously only very brief answers. You'll find the rest of what you're looking for (and like-minded addicts) in Rotorheads. See you there.
HTH,
B73