+1 to trying on shorts. There's just no other way to know what fits, and unfortunately you do get what you pay for, for the most part. Do you like the one pair you have? Can you find the same or similar shorts?
Gloves are a must for safety as much as comfort. It doesn't take much road rash on your hands to put you totally out of commission.
As far as a hydration pack, that depends. If you've been riding for years, you know how much you need to drink to avoid getting dehydrated - will you be able to stop and refill your bottles often enough? Is it a supported tour?
+1 on arm and knee warmers, depending on what kind of weather you expect on your tour.
Cycling jerseys are nice, but not a complete necessity (as long as you have another way to carry your ID, phone, key, credit cards, etc. if you don't have pockets). A good technical fabric will wick sweat, help keep you cool in hot weather and warm in cool weather, hug your body for comfort and aerodynamics in riding position, and give you some (not much) protection in a fall by sliding rather than catching on the ground. But plenty of people ride in T-shirts or running tops.
Speed comes from what you put behind you. - Judi Ketteler