Don't bother "securing" directories like that. The meaningful permission bit is the write permission on the directory holding the file. cat ~/.bashrc > ~/.bashrc.new; put-malware-in ~/.bashrc.new; rm -f ~/.bashrc; mv ~/.bashrc.new ~/.bashrc or the like will still work if you have write permissions to /home/username at all. Marking the file immutable with chattr +i as root might be slightly more effective, but realistically still not enough in a lot of cases as the parent directory can still be renamed. Not to mention you've only found some of the low-hanging fruit; your text editor most likely also has a few ways to accomplish arbitrary code execution in its config/scripting/plugin files but it absolutely doesn't stop there.
Don't bother buying old systems because they can have free firmware. Ever since Spectre, CPU vulnerabilities have made old machines completely unsuitable for high-security purposes time and time again. Not all mitigations are equally effective and with mitigations on, performance takes a massive hit on those 10 year old machines. If you can get a reasonably new system with free firmware, that's good, though.