1) You don't want the motor to support the leadscrew, so you usually mount a bearing and then you need to offset the motor.
2) Depending on how strong you're motors are, you could add a lot of friction in the system. Other than that, should be no problem.
3) Like Jim said, glue usually won't stick to the Delrin. I'd stick to the JC method of the tubing and clamps, if you're building one of his designs.
4) 100 minimum, but more never hurts. It also depends on what you're driver can handle. You might want to look at
http://www.xylotex.com. Best performance for the money.
5)I've seen a lot of machines made with drawer slides, so it will work.