While I agree with a lot of what elvis posted above, just one minor comment...
elvis said:
Pre-500 recto
Rare
Signature sound of the '90s
Tone not really reproducible with other amps
I would argue that the pre-500 Recto sound is
not the signature 1990s sound. It most certainly is not the sound of grunge or nu-metal - that'd be Revision G, produced a little later, and in much greater numbers.
Those early Rectos were
very different sounding, not dark, loose and massive but very tight and bright. It was essentially the ultimate hair-metal amp - and, IMHO, out of the box
by far the best thrash metal distortion Mesa ever created (yes, even better than a Mark IIc+ or III). But
not grunge or nu-metal sound.
Other than that, I totally agree with what you wrote. Early Mark V's are neither scarce nor notably different from new ones, there's absolutely no reason to get one instead of a new unit that is still under warranty. There's really nothing special about them. Except perhaps the low-serial mojo factor?