Looks like I have an opportunity to purchase a Butternut HF6VX 80-10
vertical with the CPK counterpoise for maybe $340 for the whole thing.
Seem like a reasonable thing to do? Decent antenna? Otherwise I have
my 80 meter full wave loop at about 50 feet. I'm assuming the vertical
would outperform the loop for DX on 75 and 40 meter bands.

If it is a used antenna, that seems overpriced. You can buy a new
one for less than that (without the CPK). You can easily make your own
radials for much cheaper.

I've used HF6V's at several qth's. It's a good antenna on 40 if you
have enough radials. On 80 it's not that great. But it will be useful in
addition to the loop.

Tor
N4OGW