Zend certified PHP/Magento developer

On checkout page validate cart items with custom condition

Looking for ideas that implement best-practices and offer the best user-experience.

I’d like to check cart items for custom conditions when the checkout page (not the cart page) loads. I’ve looked at checking window.checkoutConfig data and using ajax, but I’d rather not go this route. I know the checkout is rendered entirely with UI components and javascript, but I’m looking for an event on which I can utilize an observer, or use a before/after/around plugin on something that happens when checkout page loads – something similar to Magento 2 – Validate Items Prior to Placing an Ordeer. Also, I don’t want to wait until ‘place order’ to make this check. It would be nice to run through the cart items on page load of the checkout, check conditions, and maybe removing items from cart based on these conditions, or at least throwing an exception, and requesting the user remove items from cart.