As a learning tool, the Schwinn would be Ok if there wasn't so much wrong with it. You'll easily spend $200. in parts and have a very heavy, but comfortable bike, --IF it fits you. In the end, it will be better than that rideable Raleigh is now.
Your idea of building and learning is a good one. I think, though, that checking yard sales and the Salvation Army or Church stores, might get you something with a little less challenge and still allow for a learning experience. Also, it will get you riding the bike sooner and probably with less expense. Good Luck!
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ljbike