There's a lot of good advice here. However, I have to say that probably the least helpful is 'just do it'. It tends to be said by those who'd like a little morale boost in their own campaign, and often ignores the very real big issues influencing the original poster.
Silicon, there are no absolute answers to any of your questions. The most fundamental has to be what are the prospects for the industry over the next five years, which would cover your trainiing and your potential induction into professional flying. If prospects are bad, there may be little point in risking all; if they're good, the gamble may be worth it.
Notwithstanding the current wobbles in the stockmarket, anf the warmongering from the USA, I think prospects now are not too bad. We aren't going to see a return to the voracious employment market of 1998-2001, as the general economy won't support it, but aviation is returning to its historic path of growth. That's the good news. The bad news is that the current atmosphere is one of austerity and cost-minimising, so there aren't going to be lots of sponsorships and big starting wages on the table while there are lots of qualified and wannabe pilots looking for work - and there are. The so-called retirement bulge will open up the market slightly, but I have my doubts that BA will look to replace most of their retiring Captains with Cadets. I feel they'll be looking to poach from other airlines - which means that everyone shuffles up the ladder, but may mean that the CEP scheme remains on the back-burner. There will, however, be some improvement in prospects for low-hour pilots, particularly with the low-cost airlines.
As for the 40-year-old applicant, your chances really depend on how you come over at interview! You can still offer up to 20 years service, so you're a worthwhile prospect. As others have said, many older than you have made it!
The most important thing is that you square this with your family. You must fully advise them of the risks of failure, and you need an 'exit strategy'. That is, a point beyond which you won't go without a good chance of a job in aviation, and where you still have a reasonable chance of returning to your previous career. What that exact point is will depend on your discussions with your wife, and of course your knowledge of your current field. You're about to walk out on the tightrope, and your family is a fundamental part of your safety net. You need them 100% with you, and in full posession of the facts.
I wish you the best of luck!