We spoke to the plumbing company again last night and he said more like $4k for the furnace and $1k for the hot water heater. More depending on the duct work and electrical issues to be addressed We asked for a full quote, references, permit info, etc... We told him we'd be bidding out the job to more than one company.
That's a much more reasonable sounding estimate, but I probably wouldn't hire that company. I would still get their full quote because it's always good to be able to compare to the others, but I kind of don't trust them. Hopefully the other bids will be competitive (or lower) and you can choose the company with the best reputation.