[WernherVonTrapp]

As far as I know, WO is your sonar and SD is your air search radar. Your SD radar doesn't have a PPI (Planned Position Indicator) so you'll only hear the crewmen call out the single contacts. You won't be able to actually use the radar screen to view them. That's probably why you don't realize that you have air search radar.
As far as setting torpedoes to run straight, just don't enter the info into the TDC or, if you're on auto target, keep your periscope off the target, gyro angle 0° and fire (not sure why you'd want to do that though). Nevertheless, if the target is at 0° and auto target is on, just fire.
As far as you describe, getting detected all the time, that might be why you're missing w/torpedoes. They're detecting your presence and the target begins to zigzag. When you sound the General Quarters, your damage control team automatically activates, if you don't deactivate them before engaging Silent Running, they remain at their stations and could potentially create noise for the DDs to pick up on. When I was playing Stock, on more than one occasion, I found myself being stalked by DDs after forgetting to deactivate my DC team. As soon as I deactivated them, they DDs seemed to give up. It could be just a coincidence but, I still adhere to that rule as a precaution. It certainly can't hurt to deactivate them if no repairs are required.
There is a great significance to going deep, especially if you go below the thermocline (Thermal Layer). That makes your sub harder to pinpoint by the DDs. The warmer water tends to bend and diffuse the sound of your sub.
As far as lying still and quiet, DDs use two types of sonar detection, passive and active. Passive listens for any sounds (screws, bubbles, crewmen working, etc.), Active uses ASDIC (or sound pulses known as pinging) which sends out a sound and waits for the echo to bounce back off of your sub. If you're using the external cam, you may not hear the ping when it hits you. You have to keep your sub bladed to present the smallest possible sound cross-section that you can. That is, if the DD is approaching from your side, your boat is presenting it's entire broadside as a sound signature. If he's approaching from your front, your boat is bladed to present the smallest portion of it's size as a sound signature.
Remember, just because they're pinging you, doesn't mean they know exactly where you are. When the pinging speeds up, they're closing in on your exact position.
New boats will be offered to you as the game progresses and commensurate with your performance at sea. As far as "alternative dashboard stuff", it's all here in the forum.
You can find answers to these questions and more, at the link below:
http://www.subsim.com/radioroom/showthread.php?t=107783