eBay replacement 2bbl carbs

Metering rod piston can be replaced with a new one. The throttle shaft can be re-bushed. And "top distortion" you mean where the top of the carb mates to the fuel bowl, the surface can be sanded down on a belt sander. I have not done this on my BBD yet (resurface)...but I may need to. As long as you have all the tools, all of these should be able to be done for around $10 in parts.
New parts on a new carb doesn't mean "Better." Your better off rebuilding an old one.
Yes, much you say is true. But far beyond most for kitting carb. Have you ever re impregnated the main casting to stop porosity leaks? 50 years and modern fuels eats away at the original impregnation sealant. Estimates to get that done is a few hundred $.

Never tried the Chinese carb, but for the price, it makes a cheap test.