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Yeah 1x would be simplest but she specifically wanted an old MTB for the wider gear range than her town bike, so going 1x7 would defeat the point, and upgrading to a massive cassette/10 speed etc would end up being $$$ for new derailleur, shifter, cassette etc
That SJS page is the same one I've been staring at for ages scratching my head... I'm thinking it might be safest to look for a second hand crankset
that looks like a Shimano exage 500lx (110bcd) crank so I think you've got 46/38/24 teeth. I'm fairly sure the cheapo ones from SJS would work https://www.sjscycles.co.uk/110-bcd-5-arm/?order=price+ascending but the shimano branded ones would be better.
If it was my bike though I'd but a suitable narrow wide and go 1x
https://www.superstarcomponents.com/en/raptor-chainring-110bcd-narrow-wide.htm