What a pain. I am surprised you have to go to 200 amp. I would bet
money there's no way around that code item.

Whatever you don't make the seller do, *you* will have to do when you
sell the place. Assume that the day after you buy the house you have
to sell it for some reason - where does that leave you ?

I would make them do everything, or else deduct the full cost of doing
everything. Including the deck alteration and all that. Depending on
the height of the house I 'think' they may be able to raise the point
at which the electrical supply line connects to the house, rather than
lowering the deck. You'd want to ask someone who knows about that, esp
in your locale. To go from 60 to 200 they have to run a new line
anyway I think so maybe they could reattach it higher.

Anway, basic point - either make them bring it up to code, even if it
means ripping out the deck or whatever, or else deduct the full cost of
all the electrical and deck work from the price. The sellers know they
have a problem and making them fix it, or deducting the cost, is
perfectly reasonable. They're in the same position as if they still
had gas lighting.