Here are some close up pictures of just APUs. I work for the company that makes them for many planes but I work in a different facility that is responsible for other parts, and I have worked on electrical generators that are mounted inside the engines. Here are pictures of an Airbus A320 APU and a Boeing 737 APU. The Boeing 737 APU is smaller and rated at 60kVA, while the Airbus A320 APU is larger and rated at 90kVA. These modern APUs are only about 300 lbs. That is quite impressive for a small engine. If you have any specific questions about the manufacturing and design side, I might be able to answer them.
[Edited 2006-05-14 17:54:44]
If you have never designed an airplane part before, let the real designers do the work!