There seems to be two ways forward for you.
1. Buy a pack and then try to find kit which fits in it.
2. Buy your kit and then buy a bag for it to go in.
The benefit of the second route is that you have a better indicator of what size pack you need.
Much of the space in the pack will be taken up by your sleeping bag. Bags with synthetic fillings are bulkier but cheaper than their down filled counter parts ~ a warm bag can be left partially unzipped, one which doesn't insulate well enough will lead to miserable nights of broken sleep

. A compression set is a great help

.
Some thoughts to ponder...
Space for spare clothes ~ two to three weeks of travelling is going to take its toll

.
How are you going to source your water and food? Water filter? What are you going to cook in and on?
Availability of fuel on your route ~ learn to use your stove properly before embarking on your trip. Don't rely on being able to have camp fires!
Good boots will be a must ~ not necessarily expensive, just boots which suit your feet.
Decent waterproofs ~ do a whole load of research. Some surplus MVP (similar to goretex) is bomb-proof knocks the spots off of some of the civvy options ~ but, again, surplus kit tends to be bulkier and heavier than civvy street options.
Living in your boots might not be feasible and even a pair of flip-flops will help your feet relax and breathe at the end of the day

Repairs and spares kit ~ what are you going to carry in order to perform quick and / or proper repairs? Hole in your water proof? Zipper stitching failed on your sleeping bag? Sole flapping on your boot?
50L is a mid sized pack and although doable, if your budget is tight, you might struggle.