Pin-Guy -- interesting list in that link.
As to definition of NOS. New Old Stock (to me) means new (unused) old stock but this has been debated many times. To me - parts that are pulled from equipment are not "NOS" but actually "USED". My favorite vendor description is "Refurbished" where all some of them do to refurbish them is pull them from the old board.
Parts that I consider obsolete and hard to find for that board (e.g. 'scour for NOS parts'):
Williams ASIC -- Might be still available from the last build that the guy in OZ made... I'm not sure.
EPROM -- Those are obsolete. Can replace with an OTP-PROM (which is still made, footprint compatible and cheaper anyways).
CPU -- That one truly is no longer made with no replacement, can become a bugger as they get scarcer.
To the OP - I think the choice to use a 'point of use' regulator on that board is an excellent idea. Computers have been using that method for decades.
But I think the use of SMT for the parts that often get cooked (e.g. comparators and transistor array) is not such a good idea. If you really want to use SMT parts there, I suggest you put land patterns on the board so that when (not if) they do get cooked then they can easily be replaced with thru-hole parts that are still commonly made.
And - color code the LEDs! Human Engineering specs say 'Green good, yellow warning, red bad'. No biggie - just a pet peeve of mine.