If you want redundant domain controllers, they should be located on-site and not in VM's on the same site. Kind of defeats the whole purpose. At work, we run three DCs, two virtualized, both are backups with one physical DC, and each remote site (two) has two DCs, and either physical or virtual (one of each or both with one site)
As long as you're aware of why it's a bad idea to virtualize DCs at one site as both primary and backups, there's no reason to stop you, it's just bad practice.
But that's not why you're here..
Most decent ESXi servers are several grand, and will do what you want. You might be able to snag one used, and you don't need all that much - our primary heavy iron system is a pair of E5520's and 64GB of RAM running ESXi 4.x. The host is a Dell PowerEdge R710, and even used on Ebay, they're still $2-3k.