I've tried making models myself and it's f-ing hard .. I've giving up modding because it's a little above my head but if you wanna take go at it be my guest, here's a good tutorial:
But I'd suggest first browsing gta4-mods.com for the clothing and glasses
Converting the speech is as far as I know not possible, I've tried it myself through various ways and never got it done. This because the ivaud program doesn't open up speech files (correctly) and that's about the only change u have with editing gta 4 sound.
Good luck (it IS a great movie so who can blame you for trying.. I absolutely love that 'stuck in the middle with you' scene hehe)