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The main usage will be work/study; its hardware should be well supported by Linux (some BSD would be great).
Basic specs I have in mind:
* >= 8GiB RAM
* <= 250GiB SSD
* Processor?
* No dedicated video card
* WiFi and Ethernet cards
* No card reader
* No bluetooth
* Battery >= 3-4 hours
* >= 15 inches screen
This is basically it. My current machine is a 2012/2013 Dell Inspiron 15R, 8GiB RAM, 3rd gen i7.
EDIT: I should also add that I can't spend much more than ~ $1K