I have the opportunity to buy a used Z7 at a great price (about $450 lower than current used gear goes for) (it's not hot or stolen; the seller is a friend, who was going to sell it to a used gear buyer, but instead, offered it to me at the price he could unload it for quickly)
I'm upgrading from a D7100 and in doing so, I'd have to replace a few lenses that don't autofocus on the Z7. So that makes the purchase a lot more costly.
Of course the extra megapixels would be great, but I don't think I NEED them.
I love new tech, and don't waste any time on upgrading, but this cost is considerable. It's time to go from SLR to mirrorless.
Should I go for the 3 year old Z7, or wait a few weeks for the Z6 II?
First and foremost, you will probably be waiting a long, long time for a new Z6 II, at least a US model which is all you should buy if you're in the US. A used one may pop up from time to time, but you best be prepared to wait on either.
While the Z6 II has some great improvements over the original Z6, it still isn't comparable to the original Z7. If the Z7 being offered has a reasonable amount of shutter clicks, like under 10-15K, and the price is as you say, really good, then it really is a no-brainer.
You can do a lot more with the Z7—it renders sharper photos and the difference in noise level is insignificant compared to the Z6 II. If you are shooting everything other than fast action, like birds in flight, the Z7 is the way to go.
Now, someone mentioned that you'll have larger file sizes and so on, BUT you can shoot the Z7 in DX mode and get photos about the same size as far as megapixels go, almost 20MP compared to 24MP. I shoot in DX mode quite often with my Z7 and it gives my longer lenses that extra "reach" that you get with a D7100 and a full frame lens. It still leaves you with the option of shooting at the larger FX frame size when you need it for more flexibility to crop the final image or when you want to make huge prints of your best shots. The Z7 lacks the low-pass filter, and it has slightly better dynamic range than the Z6 II with a lower native ISO of 64. So if you want to get into landscapes, the Z7 will be the better choice.
I own both the Z7 and the Z6 II, and as far as action goes, there isn't really much difference although the Z6 II has a faster frame rate and better buffer. I haven't found either of those specs to make a big difference. If you are really into shooting a lot action, then you should be looking for a D500.
As far as having that extra card slot, poo! Yes, it's possible a card can fail, but the XQD cards that I have been using for the past couple years have never had a problem, and they are faster than any SD card I've tried so far. Yes, it's nice to have that extra slot for backup or overflow, but I have not yet found either to be necessary.
If the price of the used Z7 body is less than the price of a new Z6 II body and, like I said, if it doesn't have a lot of shutter clicks, the Z7 in my opinion is the better choice in the long run. It is, after all a $3,000 camera.
Oh, one last little thing from my personal experience: I originally bought a Z6 not long after it came out when I also owned a D850. I loved the smaller body and all the features of the Z6 so much, I rarely used the D850 after that. But, I missed the higher resolution of the D850 so I decided to sell it and move up to the Z7 which is a close second to the D850 in most respects and better than the D850 in many others. Moving from a DSLR as you are hoping to do, to one of the Nikon mirrorless cameras will completely blow your mind, and you may before long wish you had the top of the line—the Z7.
NIKON Z7 + NIKKOR Z 24-70mm S f/4.0 SETTINGS: 70mm • ISO 64 • f/4.5 @ 1/5 Sec