I'd just call 1800MyApple and order direct from Apple.
Selecting which model really depends on what your primary application will be.
Buy your extra RAM 3rd party from a reputable vendor like OWC, Crucial or Datamem.
If you are entitled to the education, developer, government or corporate
discounts you definitely want to buy directly from Apple.
You are also much safer buying a late model refurb from Apple rather than risking the used market on eBay.
The more we know about your needs, the better we can help with advice.