The format isn't that big an issue because there is free software that can interconvert (like XLD on Mac). Just make sure it's a lossless format. I do prefer FLAC because it can take any custom tag name you want.
1T is probably enough for 3000 discs, but since 2T and even 3T drives are available now, I'd just get the biggest drive you can to allow for future growth. I use an external USB drive for backup. For ease of expansion, I use LVM (logical volume management), but that's a unix thing, I'm not sure how that translates to other operating systems. With LVM I can add another drive when I need more space, issue a few commands to add that space to my existing music library, and then it all looks like one big drive.
1T is probably enough for 3000 discs, but since 2T and even 3T drives are available now, I'd just get the biggest drive you can to allow for future growth. I use an external USB drive for backup. For ease of expansion, I use LVM (logical volume management), but that's a unix thing, I'm not sure how that translates to other operating systems. With LVM I can add another drive when I need more space, issue a few commands to add that space to my existing music library, and then it all looks like one big drive.