How you jailbreak/unlock is kind of important. It's good to do this yourself, unless you have a friend nearby that can do it for you.

What OS version and modem firmware versions are on the phone now, and how was it jailbroken and unlocked?

My old (and unused) 3g has iOS 4.2.1 with modem firmware 4.26.08 and it was jailbroken using pwnage. As noted by pdxer, that's the latest version of iOS for the iPhone 3G. Modem firmware (baseband) 4.26.08 is jailbreakable and unlockable, so it's important to use something like pwnage to update the phone, which will preserve the modem firmware. (iTunes and other methods won't do that, they might update it to something that doesn't have the same exploitable firmware that can be non-tethered jailbroken/unlocked.)

-David