The best free (as in comes with Macs) DAW for mucking about in is probably Garageband, which I'm assuming you've got if you have an iMac. You can spend any amount on an interface, ideally though if you're taking a line from the D.I. in your head you'll want something with at least one XLR in socket. As Ray mentioned the Line 6 Toneport interfaces are good, if you wanted to foray into ProTools, the MBox 2 Mini is good value for money, it can also be used as an interface for any other DAW.