I follow your thinking here and see how it came about. If you have no pack, obviously you will need to buy one. If you have older, heavier, bulkier gear already, you would like to replace it bit by bit, using the old stuff until you can afford to buy some newer, lighter gear.

The basic difficulty is that trying to carry a lot of weight in a small, lightweight pack usually doesn't work very well. The heavier your load, the heavier your pack must be to have a sufficient suspension system to carry that weight in anything resembling comfort. If your load is too much for your pack you'll be unhappy and it won't be the fault of the pack.

Your first job will be rethinking what you take with you to make sure it is the most streamlined, minimalist load you can manage. This includes possibly replacing a few old items right away with newer stuff that saves you the most weight at the least cost per ounce saved. Maybe replace your old stove with a homemade alcohol stove, and remove all the cotton clothes from your list by spending time in thrift stores looking for cheap synthetic clothes that you find comfortable.

After you've done this preliminary work of rethinking your load, you can move ahead with getting a pack that best fits that load. If it turns out your passion for backpacking is reignited, you will certainly end up replacing that pack further down the road, as you replace more of your old gear and clothes and possibly plan more adventurous trips. It is a process and you're at the first steps down that trail.

Good luck.