If you don't get the information you're looking for online in the next day, I wouldn't hesitate to call an interrail agency and actually talk to a clerk about the pros and cons. Being from Canada, I've only ever dealt with Eurail passes, and I know the rules are different, so I can't give you specific advice about that.
However, I can confirm that some train trips can be wretchedly expensive (though traveling 2nd class and on less speedy/direct trains can save you $$$$$ - ie. taking a high speed train or ICE is going to cost a lot more than slower, regional trains).
Another place to check for schedule (and some pricing) information is the German train site, www.diebahn.de
. It's great for schedule info, though the pricing info tends to be a bit spotty in my experience. At least it will give you an idea of alternates to the ICE routes, if those don't qualify under your pass.
If you do decide to get a pass, you should be able to go to an agency and pick it up, I should imagine. Last time I bought an Eastrail pass, I just went into a travel agency, plunked down the money and walked out with the passes. I don't think they only have to be delivered by mail, but again, you can confirm this by contacting an agent.
Good luck, and I hope that you get more specific advice here in the next few hours.