When you say design your on ships do mean their weapons, sensors, etc? What about there looks? Can you build them like in Galactic Civ 2?
you can currently select from a list of pre-made designs for each ship. A series of designs that defaults for each race.
of course, with modding, you can pretty much do what you want. some modder on the forums has a fleet of ships based on star wars designs...
the components involve fuel, energy, hyperdrive, thrusters, and all the weapons. you mix and alter the energy components, based on the increasing size of your ship (the more junk you throw in it), and you can tweak acceleration and turning capability. There is a maximum size limit for ships, but that is increased through empire research into Industry (construction cranes).
The AI is ...OK currently. They are releasing updates about once per week right now, mostly tweaking AI behavior regarding the economy, and how much your advisers bug you. For the most part (in v 1.03), the advisers are mostly wrong every time they suggest what you should build, military action, etc. If you fully automate the economy, it will over-build mines and other ships, stretching maintenance costs well beyond what they should be. It's easy to take these things over and do it yourself, but when just starting out, there's such a high level of detail that managing each little aspect can become cumbersome when trying to learn the game.
...this is what makes the lack of demo or current pricing so difficult. The game is obviously not perfect right now, so it's probably tough for a lot of people to take the plunge when some tweaking is still needed, and the download service is less than ideal.