It is fine to leave the stock system drive, an SSD would make things a little snappier but I would not say better and nothing for stability (such youtube for SSD boot disk comparisons)
yes a WD Black like this one for sessions
http://www.newegg.com/Product/Produc...-625-_-Product
I'm a Crucial man myself
http://www.crucial.com/ but there are other good suppliers that provide a lifetime warranty
No need to pay an Apple tax
I got on a roll and forgot you were on MP not LE, I don't know if a Pro Control will work with MP.
v11 was a 64 bit rewrite/recode that lets Pro Tools access all the RAM the machine can hold. 10HD adds a RAM Disk that loads the session into RAM outside of the 32 bit code cap (32 bit code has a 4GB RAM cap, 64 bit's cap is like 64TB of RAM)
There are mixed reviews on stability. You would be looking at a much larger upgrade to