I bought a second hand iPhone 3GS which is locked to AT&T's network, I want to unlock it for use in the UK. I'm on the Redmond Pie site which can unlock and jailbreak it, but I don't know what version of iOS the phone is running. I can't figure this out either because opening Settings crashes.
I would check the serial number (printed on the box or/if it wasn't replaced, behind the SIM tray/by entering DFU mode and checking in System Profiler or using the Identify feature in redsn0w) and, if it is old enough to downgrade its baseband, restore it to the iOS version you'll want to use° and perform the jailbreak+iPad baseband and baseband downgrade procedures.
°depending on available SHSH, of course
Seeing the poor condition of the existing system software you'll likely want to restore it anyway…