As a ex-NJer, I can say the areas I lived in weren't that way. In every house we had a sink in the basement, even a half or full bath. Not sure what area you live in, but I've never heard that one before.
Moved my mother into a new house just five years ago with a slop sink, which she specifically added, and she pays nothing extra for it. Granted she pays more in property taxes, but I've got her beat in school and other area taxes here in Ohio, and I live in a rural area.
Have you looked at the portable sinks often used in fairs and other large outdoor events? They are foot pump driven with self-contained fresh and grey tanks. I've seen used ones come up on craigslist...
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here's one created for outdoors, but he's put some good parts to use that might give you more options...
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