8 thousand dollar pin with planking? I would try to find one in better shape with that budget.
I have solved planking by clear coating a stripped field, long process.
Will it spread? Probably, shows the wood was in an extremely dry (or humid) environment for awhile. In other words it swelled with moisture and then dried out.

I guess it is as serious as you want it to be. If it is an older game (90's or earlier) make sure no water damage, especially in the cabinet itself. If not, up to you if the cosmetic damage is worth your money. If newer game, I would avoid because is likely a bum playfield. But, for me, I wouldn't pay 8k for a playfield with planking. 8k needs to be mint or near mint.

Keep in mind that for some rare classic games, that price might actually be low.

From a value and cosmetic standpoint, yes. Since the surface of the wood is no longer flat, paint loss will likely continue.

Moisture and temperature swings from being poorly stored usually. The game was probably stored in an unheated/uncooled place and subjected to daily and seasonal temperature and moisture changes.

That depends on the game. If it's a common title, quite a bit. If it's an desirable and unobtainable title, it depends.

Would you be comfortable sharing the name of the game?

I used to care about planking but over time have learned to let it go and it is part of pinball. It really is hit of miss with what games have planking. Certain older Stern titles almost always had planking-Spiderman is one of them. The playfields still hold up well and really does not affect much. Just got a brand new Jurassic Park NIB that I opened up that has planking, and I let it go. I guess I am too tired to bitch about it anymore, especially now that Chas is gone and it would be a waste of breath. I am not saying it is unfortunate or that companies should not try and improve QC, but I am over it.