I built a 5508 about 8 years ago, NDTrading case and dial, various other parts and a 1520 movement out of an Air King. While the 1530 would have been the correct movement, the 1520 worked fine. Only gen parts were the crystal and the 6mm crown, everything else was rep. As I recall, I spent around $2500 building this rep.
It's hard to source gen parts for a 5508- they're scarce and expensive. Now days I'm more inclined to go the less expensive route and avoid the Rolex movement and use a good swiss eta. Aging of the rep parts can make for a nice build and when you're in the $500 range, I find it just as satisfying.
This cartel 5513 was a fun little project- shaved the crown guards and filed down the case between the guards and installed a 702 case tube and crown. Drilled out the lugholes and now it's a nice little beater for $300. Movement is the dg2813 and it's doing fine. Of course it helps when you can do most of the work yourself- all of which I learned hanging around here and on the other rep forums.