Re:
Klein (or any other retro-mtb) is in general NOT a good investment. Anything produced after about 1985 (talking mtb's here), has been mass-produced. Even for the "rare" models there are plenty around, which makes their value will never sky-rocket. I'm not talking about special bikes, like the one XX rider used to win the World Championship, just the general "I bought it in the corner shop" bike.
Of course, an early Klein will be more valuable than my 90's Trek, for the mere reason there were less produced (talking relative value, of course since the asking price of Kleins is higher, the selling price will also be higher). But neither of those will pay a pension or your kids education. And you have the maintenance costs, low, but still there (the shed to keep them, the rubber parts that go off, etc.)
Also, nowadays, most people have already figured out the value of each bike, brand and model, so the "collectability" is already priced in. The only "good deals" to be had are bikes that we see as "retro" but the seller sees only as "old", which means yard sales, local adds, that friend of your uncle, etc. but not e-bay or this forum.
I like to find old bikes which have mechanical issues or are missing pieces. I get them for very little, fix them, and then re-sell. It's not a "make me rich" scheme, but helps to support my bike hobby. "Profits" to be had are minimal, although from an economist point of view the "returns" are massive since I can get a malfunctioning early 90s mtb for 30 EUR and after an overhaul and clean-up sell it for 90-100 EUR, a 300% return. I'm aware I'm not counting my own time, which is also money, but since it's a hobby I don't care, I would be wasting hours on the internet otherwise.
I do this whenever I find a good deal, but usually no more than twice a year. But if you are methodic, you can end up with a nice side income. Plus I love fixing bikes and doing this keeps me entertained without increasing the herd.