The problem is that such bikes are quite unsuitable, depending on how much you want to overload them. A seatpost rack is a bad idea for anything more than a very light load, because it exercises pretty strong forces in the wrong place, and because it sits pretty high. You could fit a rack like a Tubus Fly and use two front lowrider bags on that rack, or a large saddle bag like a Carradice Camper longflap (with good support like a Nitto R10 rack). In both cases you are talking about 25 litres maximum, weighing perhaps 7-8 kg. That will work if you yourself are not grossly overweight. If this is for a hosteling trip you will not have a problem, but if you want to camp, you really need to know what you are doing, and focus on ultralight gear like a 1 kg tent, a 400 gram mattress, a 400-500 gram sleeping bag etc. If you want to ease the burden for your bike, fit the widest possible tyres for some suspension, and have a close look at the rear wheel. Maybe you need to buy a strong handbuilt traditional wheel with 36 spokes and a heavier rim. Finally, have a look at the gearing. So the question is: is this for a camping trip or a hostel/hotel/B&B trip?
Willem