What are the issues with adding Craft Commerce to an existing Craft CMS site?
You should absolutely use Craft Commerce to store the products, as there is no migration from Entries -> Craft Commerce products.

You don’t need to pay for a Craft Commerce license if you are not utilizing a payment gateway, so there is really no downside to getting things into Commerce in the first place, and plenty of downside to needing it and not having it already in Commerce down the road.