I built this a couple years ago (now defunct) for the same reason :) The public JSON endpoints on shopify stores make it pretty easy to get the data. You mentioned using Mongo but it sounds expensive. I honestly think you could do this with just elastic or even postgres full text search and save money.
Here's a pro tip + feature you should implement: Shopify has a semi-hidden hack where you can link directly to checkout of a product if you know the variant ID. You could add a BUY NOW button to your site without forcing the user to navigate the original site or checkout flow. Example:
https://hapaboardshop.com/cart/42165521907955
(it also supports quantities and coupon codes)
A word of caution: more products isn't necessarily better. I definitely found there to be a long tail of really bad shopify stores and products. IMO it's better to curate or audit the stores you index–otherwise you risk your site being littered with kitchy t-shirts or drop-shipping garbage.
Here's a pro tip + feature you should implement: Shopify has a semi-hidden hack where you can link directly to checkout of a product if you know the variant ID. You could add a BUY NOW button to your site without forcing the user to navigate the original site or checkout flow. Example: https://hapaboardshop.com/cart/42165521907955 (it also supports quantities and coupon codes)
A word of caution: more products isn't necessarily better. I definitely found there to be a long tail of really bad shopify stores and products. IMO it's better to curate or audit the stores you index–otherwise you risk your site being littered with kitchy t-shirts or drop-shipping garbage.