IPhone :: Iphone 2g - Firmware 3.1.3, Modem Firmware 04.05.04 - G

I gave my iphone to be updated to a 'friend' and now I am having problems in general, like lock downs during dialing and conversations.

I wanna do the best I can to first revert back the phone to original, possibly with its baseband, and then take the phone firmware to jailbreak, unlock use it.

its actually working with a different GSM in overseas, so I am assuming it is Unlocked however I am not sure if its Jailbroken,

my dream, mission is to get it back to best iphone 2g jailbroken baseband/firmware, that I would need your suggestions in terms of the best possible version

second I would need somehow what to do from scratch to take this phone all the way back from 3.1.3 to that suggested version.

really here to learn and understand your suggestions, be aware that I haven't yet connected my iphone to any itunes. I am worried something bad is gonna happen if I do something like that without knowing the extend of it.