Replacing the K&T is a decent project. $10K sounds a bit on the high side, but possible depending on how many circuits need to be updated. You’ll also have to follow up with some wall repairs as the electrician will definitely make some holes to run new cables. And I’d definitely recommend replacing/updating. Some insurance companies won’t insure a house with K&T.
In my opinion, it’s not a huge issue, but it is a cost. I would negotiate with the seller and get some kind of credit.
The furnace is another decent problem. Could be something simple, but also could be more. It’s hard to tell for what the inspector said.
The plumbing also could go either way. Hopefully just one small leak - which is probably a few hundred $$ to fix. But if it froze or something like that... it could be more than just that one leak.
I’d definitely pick a number that you’re comfortable with and ask for a credit. Worst the seller can say is no.