I can't answer your mountain bike question (which someone at a bike shop could answer), but I can help with the others.
As for what the "best" spinning DVD's are, that's a matter of opinion. A lot of people like "Spinervals" because they are tough. I frankly find them rather boring. I prefer workouts with good music that follow the beat of the music, and spinervals don't do either. On some workouts, the coack (Coach Troy, who doesn't ride himself, but rather walks around with a stopwatch telling us what to do) chitchats with one of the class members during the cooldown, and we are not privy to their conversation. I feel less like a class participant than an observer.
Another DVD series is a new one that comes with the Johnny G spin bike sold on QVC and in informercials. The workouts look good, but they are not (yet?) available without the bike.
There are really not a lot of spinning workouts available, especially on DVD. Jay Blahnik has several videos out. I really like his "Precision Cycling" (available from
www.collagevideo.com ). He also has an "Interval Workout" and "Hill Repeat Workout," both recently on sale at the Schwinn website.
Collage also has a two-workout video by Robert Sherman. His teaching style could be seen as either strange or merely intense. Collage doesn't mention that there are two workouts on this video, so it's a nice surprise!
Mindy Mylrea has a workout that has pretty bad audio and video quality. She is planning on producing another spinning workout in her next batch of workouts. Don't know when that will be available.
An alternative to DVD's and videos is a CD series (2 so far, with a third coming soon) called "Cardio coach," that is meant to be used on any type of stationary equipment. There is music accompanied by vocal coaching. Volume 1 is about 30 minutes, and volume 2 about 35 minutes. There is no ending stretch, which you do on your own. I've used these, especially volume 2, several times and enjoy them.
As for additional equipment: keep a towel handy! And padded shorts are helpful. I've gotten some on sale at
www.teamestrogen.com (comes with a cute free note pad!) and
www.performancebike.com . I also got a pair at some place called something like "three sports" or something like that (a triathelete site). Just search for "padded bike shorts" and you'll find some other sources.