I owned a 2005 Met I scooter of the type you are describing. After 8000+ miles it was still going fine when I traded it in in 2011. The crankshaft failures were stories of some merit, but not absolute failures promising to happen. Some of the bikes of that vintage had that problem. If it was all that common, there would be stories about it all over this forum from back then. There aren't. It was an isolated few scooters. And If you're not keeping the bike for all that many miles, why worry or care?
Your bigger concern for a bike that has original equipment 11 years later is much more likely the tires, which by now will be dried out and looking cracked along the bead. I would suspect your purchase will be needing new tires sooner than you think unless they've been replaced recently. It isn't the mileage, it's the age. And I'd be asking a few questions about when was the last time the coolant was changed and if the battery is new? Everything is 11 years old unless the owner can show it was upgraded more recently.
Yes the price of $600 for this old scooter with few miles is a good enough price, but I think you'll be putting more money into it a lot sooner than you want to unless you look carefully and get satisfactory answers. As for the oil leak that "never needs oil added", how is the leak manifested? If it leaks, it needs replenishment. If it leaks, you get friction and heat and that means wear and tear. So how is there a leak, but it has no consequence?
Either spend more for a better bike of a more recent year, 2006-2009 were excellent Met years with fast unrestricted bikes, or make sure you won't be adding money to the purchase while your daughter learns to ride on something with possible defects. The crankshaft failure is potential nonsense. If needing tires, a battery and a coolant flush and replacement are not considerations because the service records can show these are covered recently, then buy the bike. If any of that is going to be your next expense, go shopping elsewhere. It's not what it costs, it's what it's worth.
Buying an 11 year old scooter with a known oil leak, plausible anticipated further expenses and low mileage showing irregular use is no bargain, even at $600. And you'd better make sure it actually reaches 35 mph by road testing it. "Looking trustworthy" is not the best way to judge a nice old gent selling an old bike you're already leery of. It's the bike you have to trust, not the seller''s looks or words. Will you think he's still a nice guy when the tires go flat and the battery dies a month after you buy it? Your $600 is better spent elsewhere unless this bike has been well maintained and can be proven so. You asked for opinions, so I give mine.
EPSP #42-- First NY posse member!