you don't need an apple SSD for the system HD - just an SSD for the video content (which could be located in the optical bay)
We use the OCZ vertex range of SSD - as they seem to be the best value for performance.
Apple SSD (toshiba) are expensive and not as good performance as OCZ vertex range.
I doubt the graphics card will handle any Matrox device. The MDP is single link.
Only way to acertain results is to test with your content and setup scenario.
MacBooks are very limited compared to MacPros - however, it depends what you want to achieve. MacMinis are about the same performance as MacBook Pro - so its worth consiering as its a lot cheaper!...
Catalyst will work with Lion - so is fine for Native installs on a MacBook or MacMini. Theres no reason to downgrade. (only MacPro)
Drivers are the main issue with 'downgrading the OS - However, the hardware has not changed (as of May 2012) with a 5.1 MacPro - so is still easily possible to run Snow Leopard.
Newer MacBooks or MacMinis may not run SnowLeopard as drivers are not present for the newer hardware...
Only difference between software versions is the number of layers, mixes and DMX PixelMad outputs - so for a lower spec MacBook or MacMini you may as well save your money and just buy a 2 or 4 layer version. You can test to see what works without a dongle before committing to purchase.
hope this helps
S