Hah for a minute I thought you were going to describe some hack to get a SID working in an Amiga, that'd be freakin' awesome.
Anyway I'd recommend getting a 1200 from AmigaKit.com, they've got good prices, great support/warranty, etc.
As far as a SID, it almost seems easier and cheaper to buy a spare 64. Then you've also got spare parts (such as the PLA chip that also seems to die often).