I cannot comment on iDatalink, as I have only put a few in (they worked fine) but the overall consensus is that they make a good product.
What I can tell you is that if I were to install that alarm in that car I'd use a DEI GMDLBP. This bypass will do transponder, doorlocks (with stock alarm bypass), and dome light/door trigger for you. They're about 35 shipped on ebay and work great..
You'll need 1 extra relay for the Caddy's second accessory if you do have remote start, provided that unit only puts out 1 accessory (most only put out 1) - you'll have to look at the schems for the alarm. The rest of the install is pretty straight forward on your vehicle.
ads-al-ca flash that bish for your car and data to data comunications makes to cake! you will haver very very few wires to hook up.... also can hardwire it..... if you can flash the bypas the gmdlbp withh work fine as well justhave to find all your wires 2nd acc.