I don't think it should cost anymore to get a helical versus a straight or offset fletch. Now feathers cost more that vanes however. I don't see any reason you could not use a helical with that rest, then again I don't own one. Tons of people here do though, so someone will let you know. And yes you may get better flight with your fixed blades using a helical, it really depends on how good your arrrows are and how well you tune your bow though. Helical fletching is more forgiving as long as you have a rest that will work with it.

Better strings are always a good investment, What type of bow do you shoot now, and how old is it. Some bows come with very good strings to begin with. The better strings are pre stretched and made out of a tougher material. This means they won't creep, stretch or twist on you. Once you get your bow set up and shot in it will stay in time and your peep will roll inconsistantly when you draw the bow back. You do have to take good care of them. From what I am getting when they go bad, they just go. You don't really get any kind of fore warning. So like I said, you have to take care of them, and change them on a regular basis whether you think they are bad or not. Chances are the bad spot will be somewhere where you can't see it any way becuase it's covered by serving material. Having a bow explode at full draw is not fun. Take my word for it, I've had it happen.

Paul