I'll go a bit against the grain here and say that I don't think it's all that important who you buy from. Not to denigrate the serious vendors who want to be helpful and supply a quality product. I've bought from thatsdax, for instance, and I certainly do respect Duane. He's a good guy.
But the reason that I think you don't need to rely on your supplier is that I think you must do it alone. To be a successful happy time rider you must be a successful happy time mechanic. But here's the best part; it's not hard at all!
Start by being a decent bicycle mechanic. You've attempted two friction drives? You're probably already there. For anyone who's reading who isn't yet good at this; don't worry. It's pretty easy.
Then build your bike and spend a bit of time riding it gently and close to home. What you're doing here is getting a feel for your bike. What sort of treatment it responds well to and what sort of treatment it doesn't.
Then make use of the wonderful treasure trove of information to be found right here at mbc.com. For instance, when the day comes that your engine just won't start you might type "magneto troubleshoot" into the "search" box. In no time at all, you'll be able to verify if your electrical system is working properly. If it's not, then you'll know just which component has gone bad. You won't have to spend time and money trying, say, a new CDI only to find out that that doesn't fix your trouble.
Buying from just anyone, you might occasionally get a lemon. But who cares? You've got $150 worth of spare parts right there. Just buy another kit. At least, that's how I handle that. I'm on engine #4 right now. If #5 turns out bad, then those parts will be just fine with me and I'll buy engine #6. (This is a bit inaccurate. I've already bought engines 5 & 6. 5 will go on my wife's bike soon. 6 is our spare. But you see what I'm getting at. They're cheap enough that having a spare on hand is affordable. And being prepared to buy another at a moment's notice is no big thing.)
These engines and kits, for that matter, are so inexpensive that I see no point in tearing into them. I certainly do respect those who do tear them down and rebuild them. And I do find the idea interesting. But I just don't see the time and effort paying off. I've got higher priorities. (Maybe when I'm retired.) If an engine dies, I'll get a new one.
As far as hardware is concerned, it's true; the hardware that comes in these kits is mostly very bad. But most of it can be lived with for a while. Even though it's annoying. But there is one set of bolts that I'd replace right from the start. Those bolts that hold the chain tensioner to the chainstay. The ones that come with the kit just won't do at all. Trying to use them is inviting damage to the drive train. I'd throw them away and replace with good grade 8s.
Looking around I see quite a few aftermarket upgrade parts that look pretty tempting. But I also see costs multiplying a great deal. I can build a very good bike for $400 or so. I wonder if doubling my cost will get me a bike that's double in performance. Or durability. Or overall 'value'. I have doubts.
Go cheap!
To make it work, you need to know what you're doing. But you need to know what you're doing with one of these bikes anyway.