Considering the costs for upgrades you're already looking at, as well as the inherent issues of using a notebook in this capacity (cooling, fan wear and dust issues, heat dissipation with the lid closed (potential heat damage to the screen), what happens with the battery - keeping it in isn't good (will deteriorate quickly and may burn up), removing it might not be either (physical stability)), I would suggest to reconsider this plan.
Getting an old (EOL) x86_64 UTM, some kind of x86_64 (baytrail-d or newer Atom SOC) thin client or some kind of mITX based solution might not be more expensive, but is likely to work better over the long term. And this notebook can still be a worthwhile computer (aka "surf station") with a desktop linux installation.