I purchased the original Fallout 1, 2 and Tactics some time ago and they are still in my library named as "Classic".
Is there any reason to purchase the current Fallout 1, 2 and Tactics, or am I better off with the originals?

Short answer:
There is no reasonable justification for the re-purchase of the three aforementioned games.

Long answer:
Found here.

Later, I had realized that GOG, at some point in time, for the 'classic' version (of Fallout 1, at the very least), had replaced the old Interplay installers with the Bethesda versions. As shown within the MD5 checksum list, the core files had remained unchanged. The high-definition patch is the only noteworthy addition (which can be located elsewhere for the 'classic' version).