There is about 50 million threads on this topic, they always seem to get a little heated..... I have several opinions to the different parts of your topic.
First of all, I would never suggest any horse to anyone until I knew what their goals were, what they wanted. If they wanted something to show and they NEVER had any interest in breeding, than I would say look at EVERYTHING, including mares, geldings, and well-behaved stallions(stallions can be gelded).
If they weren't sure, and they thought they might want to breed someday, than I WOULD discourage them fro buying a gelding, because no matter how hard you might want or try, a gelding can not breed. And there are plenty of well-behaved and kid or adult safe mares out there. If they decide never to breed, than that's fine, they still have a nice mare.
I would discourage any newbie from looking at stallions unless their initial plan would be to geld him OR they had easy access to a trainer and/or somebody very experienced.
As far as what I think should be gelded, I try not to get into that battle. Some say the best you can afford, well some people can only afford 500 and that usually doesnt get you anything very good. Some just say "The Best" well there are several thousand stallions being used out there, and they aren't all the best. It comes down to looking at the stallion and deciding if they are a good representative of the breed. Not every horse can be the best and not everyone can afford a 6 or 8 or 10 thousand dollar animal. But is the horse a good representative of the breed? And is the horse healthy an sound? If you can answer yes to those questions than I think your fine. It doesn't have to be a national top ten to be a good rep of the breed.
As far as value, here's where I am going to get flamed, in general I DO think geldings are less valuable then mares or stallions. Just start by looking at gelding prices on websites, they are ALMOST ALWAYS (notice the almost) cheaper than their similar quality barn mates who are mares and stallions. That should tell you right there! Many Many farms will give you a DISCOUNT with a gelding contract. So as far as $$$ value, just look at the info that is out there, they are not as valuable.
Emotionally and practically, for a beginner child, geldings are worth ten times more than any stallion and many mares, but $$$$ value in the market, they are not. Beyond that, let's say a child grows out of their horsey stage by the time they are 13 or 14. Now you have a gelding in it's older teens, the re-sale value is going to be VERY small. They are beyond their show prime by that time, and people are going to think twice before buying them. But a 16 year old mare could still have several more babies, and a 16 year old stallion could still have several more breeding seasons.
Again let me reiterate this, I am not talking about what is practical or emotional value, but $$$ value, YES, I think geldings have less $$$ value than mares and stallions of similar quality.
I bought my stallion as a weanling, he is now almost 2(this spring). He gets handled daily, plenty of attention, and plenty of discipline to go with it, and he is still a butt head. He is well bred and a very nice put together horse. If he was my only option for having a stud, he would be staying a stud, and I would deal with his nuttyness. However, I may have another option soon, and I may decide to geld mine. For show purposes, it makes more sense to have a gelding and a stallion, rather than 2 stallions. But it also has to do with the fact that I know my boy is with me for life. If he was a re-sale project, I probably would even consider gelding him.