I'm a big fan of having more than the minimum. While the extra umph! might not be "needed", it will be appreciated. And the price difference is becoming negligible. Also, SATA3 is backwards compatible with SATA2 so there is really no reason not to have it.
As for your speed requirements, here is a speed chart of the current king of consumer SSD (OCZ Vertex3, SandForce controller, SATA3): http://www.anandtech.com/show/4256/the-ocz-vertex-3-review-120gb/6
Here is a chart of a test that uses Incompressible data: http://www.anandtech.com/show/4256/the-ocz-vertex-3-review-120gb/10
This is already bit outdated, but gives an idea that 150MB/sec is not a problem with good SSD and it does not have to be that expensive one.