I'm sorry if my response seemed mean. It wasn't meant that way. We get these threads all the time, I think it's a silly thought to have about a fun toy. No one should buy any motorcycle as an investment.
First. You won't own this new-to-you Buell in 20 years. Right now, you think you will. But you won't. You're the anomaly if you own one bike for that long. I'm sure someone will pipe up and say they did, but it's rare. Did you own anything for that long? Were parts an issue? It's almost 8 years since the last Buell left the factory and you can still find tons for sale for cheap! with 4000 miles or less.
Second. There were plenty made... so used parts. Not Honda plenty... but plenty.
Third. None of the consumables are Buell proprietary parts. Oil? Air filter? Tires? Bearings? Put 90K miles on it and relax.
Fourth. The list of Buell proprietary parts is actually very small. Frame, swing arm, wheels, plastics, things like that. "Electronics"?? Not on a Buell. They are universal switches, relays, wires and a rudimentary ECM that has proven to be solid state dependable.
Fifth. Think worst case scenario, whatever you like... off a cliff, engine seized up, whatever. Your insurance will pay, or not. Buy another one and sell off the old parts.
No matter what, you have to remember that its a motorcycle. It's not a family car, not an 'investment', and it's not the Mona Lisa. It's a passionate decision for a fun toy. Buy it! Use it! and let the smiles you have in your helmet decide if you made the right choice.