Which CRM would be best suitable for my dad’s small plumbing business?

My dad has a small waste water treatment business. I believe he has around 300 customers, all of which he services their system once a quarter.

Mum currently does his book work, and is manually generating a list of customers which he is to see within that month. Ideally, a CRM which holds all customer details and then generates this list would be ideal, at least for the time being!

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Any help will be appreciated:)