Becoming an Air Ambulance pilot - am I dreaming?

[Devil 49]

1. At 37, am I too old to carve out a career as a commercial helicopter pilot?
No

2. Are there reasonable prospects for employment (either part or full time) if I was to commence and complete a course within the next 18 months?
This is a tougher one, but "reasonable prospects"? Probably not, more later.

3. The ultimate objective is to become an air ambulance pilot. Would I have cat in hell's chance of achieving this and what would I need to do?
Yes. No. And- maybe. It's possible you'll get where you want to be. It's very unlikely, but it could happen. If you're industrious and the stars align, you could work in this field. Having a measure of financial security helps, as does being mature (there are positive aspects to being 37) and experienced. You could do it.

Reasonable prospects?
There's a lot of experience available and looking in the market, right now. Lots of capable pilots will be entering the market before you are competitive. Bad for you, nugget
Acquiring the required experience is a considerable challenge, many (the majority?) never get it. Commonly, one 'travels' extensively and 'lives rough' while actively beating the bushes for the next flight hour. It is tough, even with the ready necessary in the bank.
The majority of EMS pilots are ex-military aviators, and some operators seem to 'prefer' a military background- you won't fit that.
EMS seats aren't predictably available at a preferred location, and that's a major draw for me and many of my peers. I could fly more and/or be better paid in other phases of the industry.
The 'good seats' are very, very stable. Hint- many of the industry's open seats are open with good cause...

You could do this thing. But, it's not a 'reasonable prospect', more the 'cat in hell's chance' kind of thing. If failure's better than not trying- well, somebody's going to be filling a seat at my base in the next few years.

P.S. Don't 'work for free'. Do favors, as required, but don't ever give it away. It cheapens what you'll be trying to accomplish, don't minimize that factor. Prostitution (conventional definition) is the only field where 'free' services might be preferred to paid. I'd like to think my profession is better than that.