1. It's not hard at all. Like I said in another thread, I'm in the process of writing up a tutorial.
2. If the computer dies, you can use whatever is compatible with your sensors and solenoids.
3. The response time is can be tuned as low as 2mS, although this is extreme overkill. Typical timer values are set around 100mS. (millisecond - 1000mS is one second)