Haha, you're assuming people have kids Very few of my friends do. Granted that's not really relevant to the masses, but still. I hadn't noticed the 4000 euro part TBH...that's just insane. I was thinking $2,000, which is reasonable. I've been managing to hold off buying a $1,200 liquor cabinet that looks like a turn of the century German light bulb tester.
I think it's a great idea, but a very, very niche one, one best left for say a weekend hobby more than a full time this-pays-my-bills career. Logistics are a nightmare as far as shipping something that heavy goes, and shipping it properly so that there's no damage. I think high end desks are fine, but as far as something costing $5,000 USD? That's luxury territory, not just high end. At that point, you could easily contract out to a carpenter and have them build you something of similar quality. A high end desk, you'd probably want to price at around $1,500 - $2,000. Some people are paying around that now for a standing/sitting desk. I have friends that make $150k+ a year that wouldn't spend that kind of money on a desk. Then again, if you're looking to sell maybe 25 a year, it doesn't need a huge market. There is a niche market for everything. *shrug*
Really, when it comes down to it, you have to try it. Take the risk.
I think the biggest issue you're going to have, is the fact you're just doing the design work. Which really is the easiest part. I could make a list of what I wanted, and a carpenter could whip something up quite easily. There's nothing proprietary about what you're doing.
more air flow always better... 3 pin have 1 RPM pin that isn't needed for the fan to run, so in theory, yes a 3 on 2 would work, you could swap the connector from the dead fan on to the new fan, I did that with a network switch box fan once, simple.