I'm not a purist - my bikes are for riding, and parts that are used wear out and get replaced. I am cheap though, so often the replacement parts are already old when they come to me. My '56 Mercian is built with mostly '70s Campag Pista.
That said, I like 9-speed drivetrains now that HG50 cassettes are 13 euros each, and I wouldn't have any hesitation about putting modern rapidfires on an older bike that's getting used. The current XTR 970s look lovely, and if they weren't outside my current budget I'd buy a pair just to have in the spares box. Shifters aside, the XTR 960 group was quite a looker.
Thomson parts in silver are lovely, and the family name is another reason to buy them.
Race Face make some very nice kit. I'm actually tempted by a silver Thomson X2 and Race Face Deus XC combination for my '94 Kilauea, miserliness permitting.
Flite SLR saddles do it for me.
Surly sell a lot of modern kit that's functional.
I actually prefer the newer Magura HS33 levers to the older ones.
When Mavic stopped making the X618 I bought two boxes. For my money it's the best all-round rim they ever made. Discontinued, but not really old enough to be retro.
My '94 Kilauea came to me second hand. It had been bought from Avon Valley cycles and was replaced under warranty with a cracked dropout. The dropouts were replaced with track ends, and it became the AVC Rohloff demonstrator until I bought it as a bare frame.
'94 Kilauea frame
Threadless P2 TB
Record/Dia Compe mongrel A-headset
LOOK Ergostem
Modolo Navajo Ceramic 26.0mm flat bar
XT M739 brake levers
XTR 960 front, XT 750 rear V-brakes
Schmidt dynohub 32h, Mavic X618, DT Competition
TA Alize Piste 172.5 cranks, WTB greaseguard BB
Mavic 631 chainring 42t
Some slotted Sachs chain that was lying around
Thomson Elite silver seatpost, SLR XP saddle
XT M732 rear hub 32h, Bontrager Mustang ASYM rim
Shimano Uniglide sprocket
One DMR chain tug
Those are parts from about 1990 (sprocket, chainring, rear hub) through about 2005 (front brake) on a '94 frame. It's not going to win bike of the month, but in my book they're all fine together.