So are you saying for the flywheel the OEM would be better @ $900?
Not necessarily, that would probably fall under paying too much for the same quality as the $450 one from idparts.
My concern was that rockauto has one for $130 bucks and a bunch for $350 bucks and there's a reason the pricing is like that. I don't know what it is, but it's either missing parts the others include, the quality is not the same, or perhaps it's just a fantastic deal, which can happen but you gotta know what you're looking for and what you're looking at before you can be certain you scored a deal at all.
I only glanced at the link you posted but it looked like they were all LUK, or at least claimed to be. And therein lies the rub: you never know what you're getting in these situations. Rockauto has you selecting a product and then a warehouse from where it'll be shipped, but you don't really have a good handle on the source of the item other than what's stamped on it.
So your friends may have had good experience, and I personally have not had bad experience with inexpensive things, like rotors for a geo metro, and "OE" plug wires for my BMW. That said, it doesn't really tell us anything about Rockauto as a source because they are simply transferring an item from their sources to the customers. I mean that LUK might be an acceptable replacement rather than Ducky flywheels, for example. My comments are an indictment of the source, not the vendor reselling them.
So you have to look beyond the vendor and read the small print about the manufacturing claims. But the real problem, imo, is that you also have to look beyond the small print. In China, remember these parts are made a country where the employees don't share a cultural or legal backdrop where copying ideas/manufacturing is wrong or illegal. Many are illiterate and all of them are going to be incapable of reading/writing English except by some amazing fluke in someone's life.
So you have an unscrupulous person contracting with an unscrupulous factory owner and they provide the team of employees a bunch of metal and stamps. The stamps say, "Genuine VW, Made in Berlin, #numbers" but they not only don't understand putting the stamp on the part is inaccurate and wrong but they don't even know what they're stamping on the part. It's the same as my toddler looking up at a street sign and telling me, "look papa, ABCs!"
She knows there's a language and is making sense of letters but in so far as what they mean or that they all aren't simply "ABCs" is still a foreign concept to her.
I contrast that with some of the more artisanal vendors here on this site who (some, at least) have stated they physically check the variables of machined products like this and reject ones that don't meet expectations. That's at least part of the rationale for somewhat higher pricing. I'm not sure if IDparts or kerma do that, but it's a simple question to ask (what's the source of your flywheel and what do you know about its manufacturing characteristics?).
If IBW, for example, PMs you a response explaining the supply chain history and any manual evaluation they do, that might be worth the extra $75 bucks or whatever to you. It's probably not worth $500 more (or more importantly, it's that 1 month wait more than the cost at issue) for VW to do all that (and it's questionable they do all that anymore anyway, but at least with them you can throw it on the table if it blows apart).
So I see the one at kerma and I see the one at idparts (I've been customers of both businesses, I don't have a strong position on either one other than both seem to be run by enthusiasts who would have similar concerns to what I'm voicing here and what you might be considering when deciding what and where to purchase) and then I see the rockauto options and right away I want to pick the $150 dollar one but I know that unless this is just my luckiest day I should *not* pick that one. Similarly, I'd reject a $900 flywheel. So I'd be looking at those $350-450 options with all of these considerations.
This is just my take on it based on decades of German auto ownership. Glad to have helped, but hopefully if I'm saying anything way outside the bounds hopefully one of the vendors or someone else more knowledgeable about part sourcing chimes in with corrections.