Without a complete and detailed list, I'll give you a possible idea on what you'd need.
1. Elk M1G (I really recommend it over the EZ8) and this is available in several packages. The one I used when I upgraded from an older Caddx NX8E was:
ELK-M1GKS - This is the complete package sans a CAN (metal chassis to mount it all in), your existing Caddx Chassis can be used if you don't mind drilling holes for the standoffs. The M1GKS comes with the low profile keypad that is to my preference. The ELK-M1GK is the same package with the larger keypad.
See this URL for the keypad differences:
http://www.elkproducts.com/products/m1/M1_Keypads.htm
Note this package includes an Elk 8Ah battery. I've seen lots of Caddx systems that had small 4Ah batteries which are not sufficient in terms of the current standards, so check that. Plus a fresh battery isn't a bad idea even if the capacity is the same.
2. Keypad - this is included in one of the packages above.
3. Elk M1G to Caddx NX548E wireless (or the smaller models if you have one with less zones) adapter cable:
ELK-WO35A
Important Note! Elk now has two versions of the main panel firmware, one is compatible with the GE wireless and the other IS NOT, it's for the Elk M1XRF2G which is a 144 wireless zone antenna system. So be careful to upgrade with the correct type.
4. EOL resistors, the Elk panel comes with a number of these but you may require more depending on the number of sensors used. 2200 Ohm (2.2K ohm for abreviation). I believe the Caddx used 4.7K ohm and that's not the correct range so you'll want to replace these with the correct value. ELK also supports several connection methods, so download the Installation guide (it's free) and choose one that meets your security needs.
5. X-10 or Insteon. If you've already purchased an X10 PLC and switches, just plug in the PLC as it's supported natively. If you do go Insteon, you'll need to choose your direction of either direct Serial interface controll, or using Ethernet and ISY approach mentioned.
For the Serial, you need the ELK-M1XSP serial interface and then the appropriate Insteon Serial PLC/PLM. Please note that while this has gotten pretty good, the best is via the ISY system others mentioned, so this would require instead the ELK M1XEP ethernet module and then the ISY 99 Insteon controler. As a huge side benefit, you'll also get a web based keypad to control your Elk, SMTP alerts and also IP based Alarm center notification. I use this in combination with WiMAX to get wireless alarm reporting and telco backup. I'd wired my telco setup to have alarm generated if someone cuts the phone line, yet my alarm reporting still happens via WiMAX.
Finally, the last important note: Buy the ELK M1G from only authorized dealers or you'll be left in the cold. See
http://www.elkproducts.com/support/support.htm At the bottom of the page is that notification.
Note, you'll have a Serial number with your Elk M1G purchase, use that to register and you'll have full access to the Elk dealer site (firmware updates, etc).