Adding in SSD, HDD, memory, and graphics cards are easy. Just follow the directions, and worst case, search/ask here on MR.
It's worth it to save on the funds. The only real reason for having Apple do it, is so that it's covered under Apple Care. Not worth it IMO, as such items have warranties that are just as good or better from their manufacturers, save perhaps the graphics card (might only have 1 yr, due to the heat they generate).
So do u think its better to buy the Quad Core model now than the 6 core model?