Transfer Pack:
You can use a standard (22") rolling suitcase if you will not need to tote the bag on any segment of your camino. Duffel bags also work well. However, if you might find yourself in the position of having or choosing to walk a day between villages with everything, then you will need a proper backpack.
I have seen people on several Caminos using the convertible backpacks with straps that can be hidden to turn the bag into a suitcase. Some even have two wheels.
Some name-brand backpack manufacturers (Osprey, North Face, Eagle Creek, etc.) make these convertible bags. I do not have an opinion on them as I have never personally used one. But this could be a good hybrid solution, as long as the convertible bag had a proper shoulder harness, hip belt, sternum strap and adequate back padding / ventilation. You need to go to a quality outdoor store where they can fit you to the correct bag for both using as a backpack and toting as luggage. I do not recommend buying online, without first being properly fitted by someone who understands the geometry of backpacks and walking.
Day Pack:
I recommend going with a hydration backpack with capacity in the 20 - 25 liter range. This should be adequate space for the filled water bladder, first aid supplies, snacks, spare socks, rain gear, sandals / crocs, cameras, mobile electronic devices, etc. Having exterior lashing points to hold things that do not fit in the day pack, or which you want outside for ready access, is a very good idea. If you do not want to deal with a water bladder and prefer to carry water bottles, make sure you obtain a day pack with side bottle pockets.
Personally, I use Nite Ize clips that fasten to the neck of reusable, disposable .5 liter water bottles to hang four from the front of my daily rucksack rig. See
https://www.niteize.com/product/Drink-N-Clip.asp This moves four pounds / 2 kilos from the back to the front, helping to balance my load and reduce the load on my shoulders.
When you shop for such a day pack, or any day pack to be used on the Camino, it should still emulate the characteristics of larger rucksacks. The day pack should have comfortable, ventilated shoulder straps with an UPPER strap adjustment to bring the back higher and tighter to your shoulders, a sternum strap to stabilize the load, and a waist / hip belt to allow the weight of the bag to rest on the top of your ilium. That is the large hip bone. Without such a hip belt, your shoulders will likely be sore by the end of the day.
Consider that one liter of water weighs one kilo or 2.2 pounds. A two-liter bladder weighs 2 kilos or 4.4 pounds. Add the weight of the other recommended daily gear and you could be looking at a 5 kilo - 11 pound load. It pays to invest in a quality hydration pack or day pack.
I hope this helps.
