Well, square feet per person is a matter of opinion. There are plenty of people who live aboard a 32ft sailboat 32' x 10', but a triangular shape equaling about 250 square feet. Others can't be on anything less than a 100' megayacht. It all comes down to how much crap you plan to put on the houseboat (furniture, junk from the basement and attic, etc...) and your personal feeling for how much space you need.
Best bet is to just go looking at them with the 2nd person and talk about what you like / don't like about each one. No different than a house.
Washers and dryers, and every other appliance known to man can be easily installed on a houseboat. They are HUGE compared to their more nimble cousins (traditional power and sail boats).
Will you move this houseboat from the dock
from time to time? Be aware they are not so great at going from Point A to Point B in comparison with traditional boats (motor or sail). They can't handle any really rough or really windy days.
As for winter... I am currently docked for the winter in NY next to about half a dozen houseboats. They are here year round with no trouble at all. They have insulated water
hoses running from the dock's water hookup to their houseboats. This helps keep the hose from freezing, and allows them to have pressurized water (hot and cold) without running a pump on the houseboat itself. In fact, I learned a lesson from them and am working on installing a pressure check valve to do the same this winter.
Also, unless you are going to be anchoring
the thing out in the middle of nowhere, the genset will be more of a backup if the power goes out or something. Most houseboats I see seem to stay at one slip for years and years on end.
If you have any additional questions, let me know... I'll ask the people next to me.
BTW: Sine these are so large, you want to make sure you have an adequate heating
system aboard. The ones next to me have huge HVAC
systems up on the roof above the 2nd floors.