D, I think what you're asking is how well are these replicas screwed together. How well will they hold up in the long haul, especially if you drive them as hard as you would a daily driver?
To answer those questions, comparing them to the original Porsches sort of clouds the issue. The most expensive replicas (Intermeccanica) are probably built as well as a new Porsche was in 1958. Becks, for substantially less, are probably built nearly as well, if not finished as luxuriously. The lower priced replicas - Vintage Speedster and JPS, for example - not so much. These last are still good value for the money, but they cost quite a bit less than the others, and here you really do get what you pay for.
But there haven't been any new factory Speedsters built in 60 years, so comparing replicas to them is almost a moot point. Any real Speedster you're likely to find has been restored or practically rebuilt from new parts by hand in a small shop - just as new replicas are. The 'build' quality is only as good as the restorer or home mechanic who did the work.
So, how will a replica hold up in daily use? Pretty well, actually, if you understand that you're driving a very old-school car. They require more frequent maintenance, and you do have to 'listen' to the car as you drive a lot more than in a modern car. You will definitely be fussing with this car a lot more than you may be used to. For some, that gets old fast, and explains why so many of these cars are for sale with low mileage
But, the other side is that they're much simpler machines than modern cars, so most things that need attention are straightforward to address. If you're handy, you can do a lot of the routine work yourself. The car will never mysteriously drop into 'limp home' mode on you. You won't ever need to make an appointment with a dealer to have the computer re-programmed.
I have one of the least expensive replicas, a Vintage Speedster, and, quite frankly, it was a mess when delivered (not all of them are, though). But even so, a competent local VW mechanic was able to rebuild it into a car that I can now confidently drive on any road, in any traffic, and take out of town on extended trips with no fear.
As you move up the food chain with these cars, brand new ones are less likely to have initial teething issues, but any of them can be whipped into shape and made reliable.
You just need to understand what these cars are all about before you dive in. They're not remotely like Toyotas, but that's got a lot to do with why most of us love them so much.