Most every opinion has been expressed on this subject, but I wanted to add my bit for all to consider.
1) Based on what it cost me to feed, worm, vaccinate, and take care of feet, that $100 weanling will have cost you $5850 by the time he is 5 -assuming that you never had to call the vet and have the ability and knowledge to do all training yourself. (If you figure out how to get a baby through 5 years without doing something that requires vet treatment please let me know) This option assumes that you either have a horse to ride for the next five years or that you have much more time than money.
2) The direction of the Arabian show world over the recent years would not lead me to a "show" breeder for my next endurance prospect. The high croups and somewhat light loins that get pinned in halter classes are not what get you to the other side of the mountain and back. Not to mention the long cannon bones required for the height that sells in the show ring.
3) Development and conditioning are very important in this sport, but the better raw material is going to yield a better end product. You can make a usable knife from rock, but the steel version works much better and lasts longer.
4) If you are going to go to a breeder for your horse, look for one that has a program based on generations of using horses-one outstanding individual does not make a program and does not materially increase the odds that the offspring will have the good qualities that you are looking for, but a program that for generations has produced horses with predominately good performance qualities greatly increase the odds of those being passed to the offspring.
5) If you are willing to pay for the "made horse`, look to the rider, trainer, or breeder with a record of longevity in their horses. I would feel much better about the long term prospects of a horse purchased from someone who finished in the top ten for 5 years on the same horse than I would buying from the regional champion who rode a different horse every year, especially if the horses from the past years where never heard from again.
6) If you want to play the lottery you should be prepared for the very real chance that you did not get the winning ticket. You might be the lucky one; however, almost all of the riders that I know of in my area that bought "bargain" horses have probably spent more time (and money) with the vet, farrier, and riding back to base camp in trailers than they have on rides.
7) Price, per se, does not guarantee a good horse: however, if you buy a prospect from a program or individual with a good track record I believe you greatly improve your chances and you are usually going to pay for the knowledge and time devoted by the breeder/seller.
8) My comments are somewhat more applicable to those who want to compete rather than just finish: however, I believe they would also apply to those who would like to finish on the same horse for the next 15 years rather than start over every few years. Each of you will have to make your decision based on your personal situation and desires, I just wanted to point out that the purchase price is only a small part of the costs involved-don`t forget time, follow-on expenses, and possible disappointment.
9) For those who buy an older horse from whatever source-unless you know the background do not assume that the hard tissues have been developed for endurance riding. These horses still need a lot of LSD to be ready to go fast, even if the heart, lungs, and muscles are ready, chances are the tendons and ligaments are not. If you are willing to do competitions at a moderate pace, you can certainly start much sooner than with a young horse, but don`t assume that strong muscles and good recoveries mean the horse is fit to race.