I say it's do-able but I have a few hesitations that I have to share...
Maybe I'm just a stickler for terms and the venue is everything maybe, but if you REALLY want to use a term as bold as 100%, I'm gonna raise my eyebrow high. I see it this way... gun makers (S&W for sure) has over decades (and HELLA for sure now) ship brand new guns that don't qualify to be deemed "100%" and without original box, wax paper and documents, "100%" is really hard to believe.
The 66-2 was heavily produced and not what I would consider scarce. That price is not horrible but I wouldn't call it a eye-popper in my circles.
I will also add (since you brought it up!) that a 4" Model 66 (K-frame) just, in my admittedly strong opinion, does NOT match up well against a 686. I've got many years and many thousands of rounds through K and L-frames and in my opinion, a 4" Model 66 is a great platform for a boatload of .38 Special and will totally carry nicer than any 4"-6" L-frame... but boy, when it comes to lighting off .357 Magnum, the L-frame is like driving around in a plush, new Cadillac compared to a zippy little Toyota Corolla.
Okay, that's not fair, but my point is that the L-frame 586/686 is a dream for .357 Magnum and the K-frame just isn't. The K-frame 13/19/65/66 is, in my opinion, a great .38 Special that can safely fire .357 in a pinch. But not so enjoyable.
$675 is not too much to pay if you LOVE that 4" Model 66-2, but if you end up deciding that it isn't the best platform for throwing fireballs, I don't see it being easy for you to get every $675 back out of it.