The i3 is new enough that an ENET cable should get you 99% of ISTA's functionality.
Here's the big things that ISTA does:
1. Read out vehicle data
2. Read out stored faults
3. Perform service functions (range extender start, sensor calibration, etc.)
4. Program the vehicle's ECUs (You can update the vehicles I-LEVEL with ISTA) <- This one requires an ICOM
5. Read technical information, such as wiring diagrams, repair instructions, part layout diagrams.
Some of the repair procedure walkthroughs assume you have an IMIB (A measuring box that plugs back into your ISTA computer), and will give up when it can't find that box.
I'm not sure what data you're trying to look for in particular, but the way to read out most basic data is to go to the ECU tree, click on an ECU, click "Call Up ECU Functions", and then go to the diagnosis scan tab in the new window. You can then select what data you want to read, and then read it out. I think one of the more useful data readouts here is the Range Extender operating time, where you can see how many seconds/hours are on it, as well as how many kilometers you drove with it.