I *believe* that any XTS-3000 can potentially be flashed for IMBE. If you're talking about doing this all legitimate, and having it done by a Moto radio shop or the depot, it'll cost you a good chunk of change.
There is a lot of controversy of whether any arbitrary Astro Saber can be updated to support IMBE. There is certainly a minimum Host/DSP version required for IMBE support. The vocoder board in the Astro Saber apparently comes in two varieties: Early ones have 512K of memory, newer ones 1MB. The conventional wisdom is that AS's with 512K can't be flashed for IMBE, but there have been several very convincing counter examples people have cited (as in "I have a 512K AS and I can guarantee you that it does IMBE").
Note that not every Astro Digital Saber is IMBE... it can also use an older, Motorola exclusive digital encoding technique called VSELP.
If you're interested in Moto radios for scanning/ham use, I'd recommend you spend mucho time at THE resource for Moto information: http://batboard.batlabs.com
My advice: There are a ton of narrowband capable, MBE configured, XTS-xxxx and Astro Saber radios for sale. Buy one that's already properly configured. You'll see several on Ebay and on a couple a week on BatLabs. My best recommendation as a fellow ham (who's gotten into this whole Moto radio thing rather recently) is to register at and hang around BatLabs until you find a AS or XTS-3000 offered for sale. You can quite easily check the reputation of people on the board, and many buy and sell Moto radios frequently.
As in all things, study up and be careful.
de Peter K1PGV