I have some back and shoulder issues, I switched to a canondale slate a few years ago and it made a huge difference to me. You don't realize the pounding road bike deal you until you ride something softer. In hindsight, I think the fat tires have more of an effect than the left fork, so I'd recommend you get something you can put fat tires on. I ride 650b 42mm slicks, they are surprisingly fast and amazingly comfortable. But I won't be winning any hill climb competitions. You should be able to get a slate 105 used with in your price range.