Will the monthly installment plan work for a refurbished iMac?

For official Apple refurbished products on Apple's website In my experience, no, it's only for new products.

You can check for yourself online before purchasing, for new products you will be offered the Apple Card monthly payments option. For refurbished products you will not be offered this payment option.

Hi, yaabooch, you’d have to call Apple Support/Apple Finance or your local Apple Store or independent Apple products dealer/vendor… there should be a link above your question which could help you…

John B

Yes I believe you can however you should call the on-line store and speak to a rep from Apple that can answer that question. You are not speaking to Apple on these forums, we are all end users just like you.