The hinges are the main focus here since you can't upgrade the frame hinge, it's a no brainer to buy the better hinge found on the D8 or D7i if you can afford it. I've used them both and the mechanisms on the upgraded bikes are well worth the price of admission just for those hinges alone. You just slap them closed and you're on your way. Other than that, I've never ridden the D7i but I did own the D8 which came preinstalled with fenders and a rack. The grips and pedals were awful so if you go that route, that might be your first set of upgrades, IMO