http://www.yorkietalk.com/forums/gen...f-ever-31.html
Go to the last page of this very long thread and the referenced link to the most recent article by the SkepVet. I like his articles because he references the actual research studies he is judging when he forms his opinion.
In terms of your health guarantee is it actually worth anything? By that I mean what exactly is this breeder guaranteeing that would be invalidated by not neutering by 8mths old? If she is warranting against genetic defects - those are not created by neuter status.
In terms of marking it is probable that his marking just might come on later when he is more sexually mature. Marking frommales is more marked especially if you have an intact female in the house - just ask any breeder lol.
My personal opinion is that for males there is no over arching health benefit to leaving a dog intact. And with some large breeds there is a decided health risk in s or n small breeds there are simply not enough studies on to be declarative but the body of evidence is none the less building.
Good luck with your decision but please do remember that owning an intact animal is an additional responsibility you undertake as an owner.