Building a bike can be a fun and rewarding experience. People who pay more then what the same bike would cost new often aren't looking for deals and are buying whatever the newest/best thing is that they have to have now. The whole 1" threadless disc fork is an interesting one and makes me wonder if it doesn't also have a rear disc, if so what size is the rear spacing. IA really quick search of the tubing shows that it is a higher end one and the tubing brand is a good one so the bike was probably a lot more originally. You'll be starting off with an older frame, and the 1" isn't that logical but I bought a high end steel in 2010 and it came 1" threadless as well, didn't stop it from being more then sufficient and keep me wondering about the need for oversized headsets on gravel, cross and road bikes.
Rest of the parts you can always search places like bikewagon, cambria, bike 24,randombikeparts, etc for a few year old parts that are new. Doesn't matter to me that my 3t carbon bar is 5 years old but still new when its 80.00 and still as good as the day it was made.
Have fun with it.