What will be happening with the ATP Test Trains (Y Sets)?
From what I've read, Y2 has been stripped for spares and will never run again, apparently they badly needed the spare parts (traction motors). Y1 it seems might re-enter service for further ATP Test work. Even after the V sets are retired, they may decide to keep Y1 running for testing purposes if it serves it's purpose. I mean...you've got an electric train, air-con, lots of room, comfort, if it's doing it's job I suppose keep it running.
On further potential candidates for preserving V set cars. Yes, mechanical/electrical/body condition is incredibly important, as is appearance and this in part ties in with "body condition". So, cars that have had evidence of repairs of collision damage, or various dings and dents/damage to their distinctive fluting, etc would not make for great candidates. Also of importance is how close to "original" condition the cars are. From there, if any of the more historically significant cars satisfy the above or are of equal or better quality than most, they can become candidates. Of those:-
The DC series only has 2 motor cars left (8032 and 8036). Out of these you'd pick DCM8032 as it hasn't had it's canopy compromised by the addition and later removal of the desto board (unlike 8036). In terms of DCT trailers, no idea, if I'm not mistaken, I think i've seen mentioned that the cars coded "DTD" still retain their cabs, in that case, I suppose DTD9032 which is paired with DCM8032 on V21 could be the candidate, so you could keep both those cars.
In terms of DIM camshaft, you have the Blue Goose and Candy cars. In terms of Blue Goose Trailers (9101 - 9114) you'd favour DIT9101 or DIT9108. DIT9101 would probably be the natural pick being the first, 9108 was part of the first official electric train to Newcastle 1984 and last passenger carrying train out of Newcastle in December 2014 so could be considered an alternate.
In terms of power cars (8037 - 8068), with DIM8037 scrapped a long time ago, historical stand outs would be DIM8038, DIM8039 (these 2 part of first revenue set, 6 cars, to use the DIM power cars in June 1981), DIM8046 which was rear power car of 2 first official electric trains (Wyong and Newcastle), DIM8055 (part first electric train to Newcastle) and maybe DIM8068 (last blue goose car). Having had to travel to Blue Mountains from time to time over the past few months and being in some of these cars, I can say that DIM8038 after it was put back into service in 2015 on V27 after being withdrawn for a time in 2012 has a vestibule at one end that doesn't have a door between the vestibule and passenger cabin. DIM8046 i've read somewhere has had it's cab upgraded (apparently only car to have it done). So those 2 changes could work against those 2.
In terms of Candy Power cars (DIM8069 - 8092). DIM8069, DIM8070 and DIM8089 are most significant. 8070 and 8089 lead the first official electric trains, respectively, to Newcastle (June 1984) and Wollongong (February 1986). 8069 was the first car of that contract officially handed over in April 1984 and was part of the first electric train to Newcastle. Hopefully, if they were to keep a second power car from this candy cars such as one for a museum display even, it could for instance allow both 8070 and 8089 to survive.
In terms of Trailers (9115 - 9138). DIT9115 most significant as being the car behind 8070 for first electric to Newcastle. DIT9116 was also part of that set so could be an alternate pick if in better condition.
In terms of chopper, you could do something like DJM8093/DJM8101, DIT9139, DIT9184, DJM8138, DKT9191, DKM8145 which would cover those numerous contracts well. DJM8101 had the unique 20th anniversary of double deck trains to Central Coast livery (blue and yellow stripes).
In terms of camshaft maybe DCM8032, DTD9032, DIM8038/8039/8046/8055/8068, DIT9101/9108, DIT9115, DIM8070 and keep DIM8089 as extra car so you save the 2 cars that lead official electric trains. If 8069 were to be in better condition than 8070 then could sub that one in instead.
For the above / is "or"
If they were to ultimately decide to keep about the same amount of cars as S set cars (12 - 14 range), then the above would be 7 for camshaft variant, 6 for chopper, which would fall in the same range.