My opinion is that all the CAAD models from the Saeco era are still perfectly good race bikes. This goes all the way back to the CAAD3s of 1998. You will add some weight as you go back in age, most of it will be in the forks. The CAAD3 and CAAD4 still had 1" head tubes, so fork options will be limited. Columbus still had a couple of models available, and I believe you can still buy a 1" Ritchey. CAAD5 and later models all had 1 1/8" head tubes.
The model number is pretty insignificant on the road bikes, more important is the CAAD series and fork model. Depending on year, an R1000 may have had the same frame as an R5000. The difference would have been in the wheels, components, and possibly the fork. For example, in 1998 the top three road models all had the same frame and fork.
You mention modern components, what crankset do you want to use? If Shimano or Campagnolo, you are better off to stick with the threaded bottom bracket shells. As you mentioned, circa 2001-2002 the top models were CAAD6 and the next step down were CAAD5. I believe the Saeco team only used the CAAD6 frames, but the only difference between CAAD6 and CAAD5 is the bottom bracket shell. If I was going to run Campagnolo or Shimano cranks, I'd prefer a CAAD5 to the CAAD6. The same scenario exists with the 2003-2004 CAAD7 and Optimo frames. Some models were threaded and some were BB-30. 2005 saw the introduction of the CAAD8 frames, but the top of the line models were the carbon/AL Six13s. I believe all the USA produced CAAD8s had threaded shells, but later Taiwan 8s may have had BB-30. I'm not sure about CAAD9s. I've owned two that used threaded bottom brackets, but they may have technically been BB-30 frames with bonded-in threaded sleeves. I no longer have either of them to check, but my 2010 CAAD9 is BB-30. It appears that most if not all early CAAD9s used threaded BBs, so perhaps they were not sleeved.
There's an interesting CAAD comparison here:
https://forums.thepaceline.net/showthread.php?t=252494&highlight=CAAD8
Here is my 2010 CAAD9, BB-30 with FSA crank and Campagnolo
