I found the choice of device rather odd for the use case, since it's still running proprietary baseband code. Open source baseband software like OsmocomBB on a compatible device[1] seems more suitable.
Yes, this seems like a very good choise, since the baseband is already owned. I've picked one myself from ebay with the filter rework modification, and I can even run my own BTS!!!
[1]: http://bb.osmocom.org/trac/wiki/Hardware/Phones