You don't always need to buy the latest and the greatest. A simple compatible BT machine and one or more chead/decent machines that can run windows and a Vmware images is a good start. Of course you will need some way for the machines to communicate on the network. A simple router can do this.
As for a good machine of course the more the merrier is generally a good bit of advice. But you don't need 8 gigs of ram. Plenty of HDD space, and for graphics most would probably recommend one that is compatible with cuda/pyrit.
This could go on for days and weeks but that should get you started.
There are also several threads on setting up home-made labs etc here on the forums.
See also de-ice and Damn Vulnerable Linux.