Originally I thought $3,500 CDN, but the more I look at that playfield, the more I think $3K CDN. You could always offer $2,800 CDN or so and see how it goes.
Tough to guage the market right now, though. Prices and demand are high in Ontario. Not sure about your neck of the woods. In Ontario, HUO is an easy $6K.
You mentioned:
"- Seriously mushroomed magnets causing divots on the playfield. The one by whiplash is so bad the ball gets stuck in it sometimes and can't be shaken out."
Those aren't seriously mushroomed. I've seen and repaired much worse. Very easy and cheap to fix. Simply take out and grind the metal flat. Re-install. Fixed.
"- Slightly broken ramps, but don't seem to affect strength or flow"
Ramps don't look too bad if you just want to play the game. You can repair or strengthen if necessary.
"- wear marks on the inside of the cabinet, which I would assume can just be painted"
Yep, painting/touch up wouldn't be too hard. The playfield, on the other hand, will be tough to colour match. I'm talking about the wear around the magnets. You could do the best you can and then put mylar down to prevent further wear.
Personally, I wouldn't buy the game if you are thinking of doing a playfield swap. The price will very quickly add up to the cost of a nice HUO example. Could even be more -- playfield and ramps on this game are expensive. Plastics are also very exspensive. You didn't mention plastics, but I guarantee some plastics are broken. I bought a routed IM in nicer condition and it had broken plastics.
If you want to play and enjoy the game, buy it and restore it to playable; otherwise consider buying HUO.
Cheers,
Duane