Genghis,
I would agree with the thrust of that post. The key point is that the ability to conduct a given flight test role is a product of both relevant experience AND appropriate training. The balance between these will vary between individuals, and must be judged accordingly. Some of the best FTEs I have worked with never attended a formal course!
InquisitivFlyer,
I think you've had a reasonably well-balanced overview from most of the posts above. You can probably spot the very few that have no relevant experience to offer you. If you have a real deep-rooted interest in how things fly (and making them do so better), then it can be a great career.
To give your morale a bit of a boost, I can tell you that the test pilot group of 'a major manufacturer of airliners based in the south of France' is made up of about 50% production test pilots and 50% experimental test pilots. Six nationalities are represented, and two, in fact, are female.
The majority of the production TPs were trained under the auspices of the company (or by their previous companies) to an accredited standard, having gathered previous flying experience of various kinds. Several started as engineers.
Of the experimental TPs, about 70% trained through the military long course route (both heavy aircraft and fast jet backgrounds), and the rest via progressive upgrades with the company, again to accredited standards. All are approved by the French DGA according to their qualifications and experience.
Two more points:
You need to be prepared to re-locate internationally, perhaps several times. You've heard about the UK and US industries, but companies such as Pilatus have busy flight test departments too. What about helicopters?
Secondly, I don't think we've answered your question re your medical issue. To undergo training at one of the military schools, you would need to satisfy the medical requirements to fly in their aircraft. However, these are likely to be less demanding than for those joining the military for a full flying career. For the other schools and for later civilian work, expect to need a Class 1 medical; if you can achieve that, you should be fine.
In summary, it's a great career (but never just a job) and it's achievable. But 'selection and maintenance of the aim', commitment and enthusiastic hard work will be needed, until the day you retire! To sustain that, you need a true interest in what you're doing. If that is you, then working as a design engineer, test engineer, FTE or test pilot should all appeal, and you may end up doing any, some or all of these roles. If your username is descriptive of you, then all to the good!
Feel free to PM me if you wish.