Im going to get this game. I want to know if it has trophies, and what else you can do after you have completed the game.

Yeah there are trophies, and once you complete the game you can complete it again on other difficulties (easy, normal, hard and crushing - which is really hard). Getting the trophies also nets you in-game medal points, which lets you unlock bonus cheats/skins. Pretty cool really.

I read that you have to update a patch on the game, to get trophies, but i dont have online. If i get the platinum version, will there be trophies already on it?

Well there's no difference between platinum and original except for the casing

So yeah you'll still need to update it.

If you do get it, the trophies are all doable. The toughest one to get would be the beat the game on Crushing difficulty trophy. To get the hidden treasures just use a guide while playing through the game, that's what I did. I used IGN's since they have free guides. Cheaper than paying $20 lol. Not trying to advertise or anything.