I would say getting through the training is the easy part. In my experience, even those of below average ability can eventually limp over the line and get their fATPL. I would put the odds of getting your licence as 1/4. With enough time, retakes, and money, you will get there. As we speak, the job market is really good. So airlines don't particularly care about ATPL Theory fails & averages, and so on. At the moment, even those who scrape through their training are getting airline jobs with comparative ease. But, in a poor recruitment climate, these people will always struggle getting their feet in the door.
However, getting through your training with style is obviously much harder. I would put getting all first time passes with a 90%+ ATPL Theory average, and first time CPL and IR passes as 6/1. When the job market slows down, and recruitment starts to thin out, airlines can afford to be picky. So they start to look at the above. When recruitment gets slow, you need to do everything you can to stand out.
As for finding a job, I don't think anyone will ever know how many people apply for one job. I heard rumours that Ryanair on average only take 2 out of 8 people on their selection days. But still, we never know how many people the airline intends to take, and how many applied in the first place. A very educated guess from my side would be at least 10 people apply for one low-hour airline job. That's a guess. Nine get rooted out during the initial screening, aptitude tests, sim assessments, group tasks and interviews. The remaining one gets the job. That's the very best-case scenario, in a period of heavy recruitment. In times of slow recruitment, my guess is that it could go up to 20, 30 or even 50 applicants per one job. The creme-de-la-creme schemes probably get in excess of 100 applications per position.
I'm a firm believer that the cream will always rise to the top. With a decent training record, good CV, positive attitude and a bit of panache, you can easily put yourself in the top 20% of applicants. If you can do that, you're no longer competing with nine others, but more like with two or three. Like it's been mentioned, a sizeable chunk of applicants will be duds who are eliminated in the early stages. If you are one of the "unemployables", then I would put the odds of you getting through the training and finding a job at at least 100/1 (I met an instructor who had applied to in excess of 100 airline jobs, with no success). If you're the cream, then that would be 5/1 or even less. After a couple of applications, by law of averages you will find a job.
A very philosophical way to put it would be to ask yourself what are the odds you put on your own success? How employable do you think you are? I can speculate the odds from my own experience, but I can't tell you that. Unfortunately with the timing of your training, it's likely that the recruitment climate will have slowed down by the time you get your licence, and competition will be stronger. But, you can't keep a good man down. Barring a recruitment nightmare, like in 2001 or 2009, by backing yourself and staking your claim, you will still find a job.