The Audiophile 2496 is a solid card, no issues. I have not used the Soundcraft mixers but I have heard good things about them, nothing bad. Don't know how good their A/D/A conversion is. Generally speaking you want at least 88.2Khz. Note that because the Soundcraft outputs S/PDIF all conversion to the digital domain takes place in the Soundcraft. Can't speak to the iMac (I'm a PC guy from WAY back... started on DOS 1.1) but I presume that you researched the use of the Audiophile in the iMac. As far as the Samson power amp.... I am sure it will work fine.
If you are looking for validation you are most likely in the wrong place. Ask what the best system for sound recording is and you will get more answers back, all different, than there are letters in this post. This will work for you I am sure, go for it and enjoy.