Hi fellfell. For the Daytonas... I have found that the only way to get a good one is to build it from parts. It is very expensive, yes... But the result is more than worth the effort, and for the 6263 and 16520, it is very possible to find parts, as they are not all proprietary Rolex.
For the 16520 El Primero- You need a good case. In my project, I was lucky enough to find a genuine case, which fits the movement perfect. I am not sure if there are good aftermarket cases available- I have sen some things that would indicate these exist, but I have yet to see one. The next step is to get the dial, hands, and a good El Primero movement, which can be found on eBay, but sometimes you'll need to use a donor watch to take the movement from.
For a 6263, it is best to start with a DW, or to obtain a V72 case kit, which can sometimes be found on eBay. Usually the case kits will have almost everything you need, except the movement and dial (though each set differs; check the description of the contents prior to deciding on a purchase if you go this route). From here, you will need to obtain a dial and movement, and assemble. Or, you can get a pre-made DW with a V23 copy movement, and install some gen parts on it. I myself went with a complete DW, and swapped the copy movement for a genuine Swiss Valjoux 23 movement.
So, short answer... If you want a good Daytona, it's best to build it using high quality parts (or genuine parts when you can find and purchase those items). Difficult? Yes. But not impossible!
Hope this helps...
Best,
R