How fast?

The speed of a CDROM drive is designated by a number such as 32X, 40X or 52X, which is a measure for how many times faster these drives are than the first CDROM drives ever released. A 1X CDROM drive transfers data at a lowly 150 kilobytes a second, so a 32X CDROM drive transfers data at 4800 kilobytes per second. However, there is more to it than this, since modern drives only achieve their claimed transfer rate at the very out rim of the CD. This is because the outer edge of the CD moves faster past the laser than the inner part of the CD.
There is no real saving in buying a slow CDROM drive, so anything from 32X and upwards is recommended.

IDE or SCSI?

The simple answer is IDE. There are, of course, exceptions to this. For example, if you have a SCSI controller in your machine, buying a SCSI CDROM will give ever so slightly better performance. However, for the vast majority of users it is simply not economical to invest in a SCSI CDROM drive as they have very little to offer over IDE drives.