Do you really need a modern spec? I ask because I found recently you can build a half decent machine for ~500USD using a used T440p Thinkpad. You can upgrade to the following specs:

• i7 4700mq 2.4Ghz 4 core CPU
• 16GB DDR3 RAM
• 1920 x 1080 full HD IPS screen
• Up to 5TB storage

Pros

• Supported by Coreboot
• Intel ME can be disabled
• Easy to open up and upgrade, repair etc.
• Sturdy, well documented online with an enthusiastic community of users
• Cheap
• Works fine and is even quite snappy for browsing, email, Sparrow Wallet, simple programming, Libre Office docs, simple virtual machines

Cons:

• Not going to work well for generative AI, advanced virtualization, modern games
• Bulky and noisy by modern standards
• SD card reader appears not to be working in Coreboot.

You can get the parts easily on ebay, aliexpress, Amazon.
You will need a programmer to flash the BIOS chip to install Coreboot.
Recommend upgrading the touchpad to a Synaptics T450 touchpad.

Here are some good online resources:

Unofficial guide to install Coreboot:
https://blog.0xcb.dev/lenovo-t440p-coreboot/

Ultimate buyers guide for T440p
https://octoperf.com/blog/2018/11/07/thinkpad-t440p-buyers-guide/

Another upgrade guide:
https://seiba.gitlab.io/thinkpad-t440p-upgrade-guide/

Buy ready built machines from the UK:
https://minifree.org/product/libreboot-t440p/