Get different prices when buying or selling weapons and ammo

In regards to the first start and the choice between of Ag and Highpool the answer is yes. If you do not go to one of them in a reasonable amount of time both of them will be destroyed and you will be in a different main quest storyline where you must set up alternative repeators and radio towers. But one of them is going to be destroyed anyway since you are just 1 group of Rangers. After this, it does not seem to be a time limit and consequences for not following it.

I would advise you to go ASAP to one of those locations. They are not that fun in comparison to others IMHO, but they give valuable scrap,exp and other things.

There is a quest that gives you a +4 radiation suit, to start it go to the prison camp and find Red iirc. then go back to RC and follow quest from there.