A 100 miler is a lot more than twice as hard as a 50, and this has to be reflected in both your preparations, and willingness to endure mental adversity in order to finish. Your typical UK 50 miler has an attrition rate of 10%, while a 100 can easily be 40%. Finishing a 100 miler is the art of not being one of the 40% who DNFs. It is not a question of whether you will face a serious psychological challenge, it is only a question of when. If you are a person who withdraws from a race when they are outside their comfort zone, then this distance is not for you.
I don’t know anybody who will sell you a gun, and I know better than to ask what you want it for.
100 mile runners are not supermen, and bullets don’t bounce off them. These races are done by a coalition of the willing, and most of the participants are essentially ordinary runners. Anybody can have a bad day and DNF a 100 miler. A few people her have had a disaster in their first 100, and have later gone on to be very successful over the distance. A DNF has to be treated as part of the learning curve.
I think that a lot of people who DNF their first 100, essentially bring the 50 mile mindset to the race, which is a bit like bringing a knife to a gun fight. A 100 is a very different animal, and has to be treated as such.