I would lean towards the 350 twin. The 350 Four was only built for 3 years whereas the twin was build for something like twenty.
Did an ebay search for CB350 and got 561 results.
same search for CB350F..... 67 results
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I'm partial to the four since I'm restoring one, but the twin will be easier to find parts for, not to mention it has better low RPM performance making it a little easier to learn to ride on, or for new riders.
I like your idea of the CB175 if you can find one, or even a twinstar 185 or 200. Check out Seaweb's "13 year old daughters bike" thread in the open forum to see a nice twinstar 200.
Cheers, Matt.