You can run a free OS pretty effortless, but when wanting 100% free software, you have to dig deeper and replace the proprietary BIOS firmware.
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I mean... Depends what you mean by 100% free firmware... If you mean only the boot firmware, that's the case for PCs like the ThinkPads T400, T500, R500, W500, X200, as well as the Dell Latitude E6400. Note Libreboot even recommends the latter for new full libre buys, as it can be software-flashed without disassembly.
But if you mean 100% free including EC firmware, wireless firmware, and disk firmware, then this will probably never happen, or at least not until a very very long time.
What I'm trying to say is that it's an uphill battle, arguably pointless too.
Before going with the current 30 series, I was using X200 and X60. They're both good machines, don't get me wrong. However, their age shows when trying to do modern tasks, even something as simple as web browsing.
The X60 doesn't even have the hardware acceleration capability for my usual KDE setup. By the way, you'd be stuck with DDR2.
The X200 is much more capable than X60, but try to browse most modern sites and you'll feel the machine getting hot. You could turn off javascript, but then you'll be missing quite a bit of functionalities. I definitely wouldn't run VSCodium on it for work. I'm currently using this one as a testbed for distrohopping.
To me, the 30 series is a sweet spot. The Ivy bridge is not too old for demanding computations of modern days. If you opt for the highest tier i7, you could beat a lot of the average ones from the following generations. If you don't get the processor you want, you can always replace it since it's socketed, at least for my W530, which should apply for T430 &T530 (not X230).
You might want to ask yourself: what are you trying to achieve, and more importantly, how can you measure what you've actually achieved? No, blindly following online articles is not a good measurement.
I found out later on that I had no way of actually verifying anything with libreboot. The build system is a pain in the neck to follow thru. I then tried doing it with coreboot upstream, and my experience building with it was much better. Even with it, I wouldn't have the chance to look thru every line of code, I still need to just "trust" somebody.
You can definitely play around, but if that's all you do, you'd be asking yourself why you did all that when you get bored.