I think you'll be glad you got it. I think $20,000 is fair. It being a 4-speed rather than a 5-speed like the registry said is unfortunate but it's not terrible. Many people prefer a 4-speed when not on the highway. The details about the paint job are a bigger negative. I didn't see the picture of the heater fan (it hangs off the front of the front trunk by the radiator. r.d. enterprises has the fan motor for $200 so they are available. And as for the cracked exhaust pipe, JB Weld is not a proper repair. I would think you can get that welded at a muffler shop.
Given the negatives: paint, heater motor, and the exhaust tube crack, I think that argues for a discount in his asking price. However, Hagerty values a 74 Europa in GOOD condition at $20,400 +10% for a Special. I would add something for the weber head - at least $500 and likely more. Even without the weber head, Hagerty's valuation gets you to $22,000. I think that assumes expected paint quality, repair, etc. so you deserve a bit of a break for those things but it seems Hagerty pretty much agrees with the seller on the price.
Ok, I'm rambling... Let us know how the sale goes.
Good luck!