If you want to hook the G4 up to an amplifier you need an external sound card. There is an option for one sold with the G4. It is 5.1 capable and DVDs should play with whatever mode the sound card is set to assuming the DVD supports it.
You should be able to change DVD player if you can find a new one that fits, but its not possible to order it without the DVD. You should be able to add whatever standard computer hardware you want to the G4 after you purchase it and it should work.
The major initial bugs have already been fixed and the new G4s are now shipping with version 220.127.116.11 so it should work well out of the box.