Wordpress - WooCommerce store with ~30,000 products
The limitations of WooCommerce are tied to the limitations of WordPress ( products in WooCom are simply "posts" in the database), which in turn is tied to the limitations of your hosting platform.
Magento is a resource hog compared to WordPress, which requires far less overhead. I am migrating a customer off of Magento and into WooCommerce simply because he's having to spend too much money on his hosting due to the resource requirements of Magento.
There's no problem at all have 30,000 posts in WordPress or WooCommerce. What matters is how much traffic your site will receive since that governs how much database activity takes place in loading pages on the site.
I work on a social networking site right now that uses WordPress and WooCommerce. The site has well over 250,000 posts in the database. The site runs fine on its own dedicated server.
I hate to mention this on Wordpress.Stackexchange, but I would go with using Magento in place of Wordpress. I love Wordpress and use it on almost every website that I build, but it was not specifically made for e-commerce. Magento handles just about everything better in regards to e-commerce.
I only tend to use Wordpress for e-commerce when the products and store are fairly simple. Magento will take more work to build the site initially, but I believe it would give you the best results with the number of products that you have.
I would tend to agree that you should be able to get performance out of woocommerce suitable for your clients needs. If they have 30,000+ products, they will be happy to spend, say, $300 per month of a fairly chunky dedicated box. You'd probably do that for Magento in any case. Cache everything cachable, and you'll be alright!