John, I Didn't include WooCommerce because of the new pricing structure and because MarketPress does pretty much the same without the extensions though MarketPress lacks a grid view and product slider. In other words, no need to pay more for authorize.net and other payment gateways.
WP eCommerce is the only WordPress plugin I know of that has an extension for minimum and maximum expenditure/order quantity values. There is a cheeky free plugin in the WP repository that opens up gridview layout, product variations are really easy to add (I do mean EASY) and there is a free plugin that automatically switches thumbnails when a variation is selected. It isn't Magento but it is as close as you can get. It's well supported in terms of free extensions and the upgrade to the paid version is less than $50 USD; though that is per site.
Another eCommerce plugin is based on OS Commerce. Forgotten its name. It is a powerful system but is essentially OS Commerce embedded into WordPress - it's not nice to use. Magento is the better option.
An up and coming eCommerce solution is Ready! eCommerce. The developers of this plugin are actively looking for ideas and suggestions for development.
I agree with Nick, you won't find Magento in a WordPress plugin but you can find ways to integrate the two platforms. Try Magento in a LAMP, WAMP or MAMP environment. Add a few test products and you'll see how awkward it is to use when migrating from WordPress but also how capable it is of selling. Few WP plugins offer the ability to feature cross sell and related products to prospects.