Some links from you to look at
Open source 2G and 3G stack: www.openbts.org
My recommended SDR: https://www.ettus.com/product/details/UB200-KIT
Suggest getting the GPS disciplined TCXO option, the oscillator built into the B200 is suprisingly crap for a $560 radio. A $400 hackRF board will work but not quite as well.
Rasberry Pi to drive it will be ok as proof of concept; for real work an old i5 quad core laptop with USB 3.0 and a few GB of RAM is more than adequate.
Roll your own femto cell, do intercepts of SMS, whatever. Note that rainbow tables for GSM A5/1 crypto are available if you've got a few TB of spinning rust available and feel passive aggressive. Note I do not condone intercept and decrypt of messages not your own, etc, yadda yadda.
I built a femtocell a couple of years back with openBTS and an Ettus B210 because I needed a comms net in the middle of nowhere and equipping my people with 2G flip phones was the right answer - we had dozens lying around. Total cost was maybe $3k in materials and a man month of labor. Compared to the cost of deploying an APCO P25 radio net, the cost was in the noise.
Friend of mine teaches high school students about risks of online life and whatnot. She uses a beta version of the UMTS fork of OpenBTS to scare the hell out of the students by sending them uninvited SMS... Displaying all IMSI in range in real time, etc. Good clean fun and (one hopes) makes them think about what they expose online, so to speak. Yes, this IS done with informed consent; she's a white hat.
Bottom line? This genie won't fit back into the bottle. Not unless we develop truly secure standards and protocols for telephony, and what government wants that???