Can't remember where I got the Guide mission from. I guess it just pops up after you've done enough of the main quest. In other words I don't think you can get to the point where you can reach Pripyat but not already have the 'Meet the Guide' mission.
You can't get the decoder unless you meet the guide and then meet the doctor. The stash with the decoder in (in the hotel in Pripyat) is empty if you don't have the quest to find it. Same with all the stashes in the game, they're empty until you find the PDA telling you about them. Kinda silly but there you go.
Everyone should see the 'green fields' ending btw since as I was saying earlier this it the only ending that lets you play the final level, which is absolutely awesome and the best piece of 3d modelling I've ever seen. Without going into spoilers if you do the 'wish granter' or the 'join the consciousness' endings you miss out on this final level. Having said that it's still worth making saves and seeing all the endings that are available, and the reactor core with the wish granter in it is well worth seeing too.
Real shame the game isn't what was originally planned since the good bits show how awesome it would have been with a much bigger zone, vehicles, sleeping, fatigure etc. Could have been the best game ever.