If I just bought the game, I would go straight to TA.  TA has everything from the earlier games plus more.  There have been so many upgrades to graphics and gameplay that I could never go back.  For example, just take a look at how few customizable options there are in the game startup screen in DL (vanilla) compared to TA.  I assure you, you will not regret it.  Also, if these are your first few games, stick with the Terrans for a little bit.  They have the easiest tech tree to understand since all the races now have unique tech trees.  If you still have trouble wrapping your head around the unique tech trees, you can go back to DA.  Just don't stay there for too long.  You will miss out on all the good stuff.

I really can't answer your second question.  I have never randomized the personality of the races.  But according to the manual, all you can randomize is the intelligence levels and who you will be dealing with.  Drengin are born mean and nasty and they will always be that way.  (Unless you get the random event where they decide peace and kindness is the way to go.)

You can also probably create a bunch of custom races so you never know what you will get.

TA

On the screen where you pick your opponents theres a little box you can check to randomize there intelligence