Hi there,
Assuming your BB course is the same as ours, the standard thing we tell everyone is to assume 500-1000 rounds of ammo (depending on how many warm ups the instructor would do, depending on how much of an issue you'll have with the various stages, etc). Re was it hard - it very much depends on individual people. Being good in one type of shooting, even other type of PISTOL shooting, doesn't guarantee that ipsc will be easy (a couple of years ago the top bullseye woman shooter in Canada went through the BB course, and she did not have an easy time - and she is GOOD in bullseye). You can never predict which stages people might have issues with, you just give it a try and see what happens.
if you can keep your shots on an 8.5x11" paper at, say, 15-20m, that's pretty much all the accuracy you'd need, getting into the course. The instructor will take care of the rest.
Make sure you have good equipment, you can reach the safety (if any) easily, can reach the mag release easy, etc... If you have multiple guns to shoot the course with, choose the 'easiest' one. The gun you'll pass the course with does not have to be the one you'll actually use in matches, afterwards
Enjoy!