I have never owned a Vita-mix though I imagine any of them are good for green smoothies. Blendtec is another "high quality" brand.
I just use a Kitchenaid blender for my smoothies.
ETA: doing a quick search, I'm seeing people mention the Vitamix 5200 the most. I don't know if it's because it's just popular or what. But doing a vitamix/green smoothie search keeps showing that model.
Any VitaMix! That's what they do!
I got lucky with ebay. I had an obsession for a while watching them, and randomly bid and won one from the (I think) the 70's! It came in mint condition and works beautifully! It has a stainless container, so you don't see , but magically comes out liquid! .
I'd get what you can afford. I've seen them on QVC at good prices, and the reconditioned ones are the ones used for demos, so they're a good deal.
Vitamix is expanding it's factory, so keep an eye on their website for deals!