I think a "Complete set" would be DTI-DTVII (obviously) #1-#500. That was and always will be the set. A bonus would be LSOE <501, and a TWTTK <501 (matching is the best). Also, purely hypothetical, but if King does a re-write or a DT 1.5, 2.5 etc. Having a#500 or less will be important, but not crucial.
Bottom line is that DTI-DTVII <=#500 are the set. Other books and matching numbers are a bonus-but only really valuable if the next Grant book is cut to a 600 limitation-thus the Garnt TWTTK <#601 will be in demand.