For years, the Chinese government has owned a substantial portion of Lenovo (28% as of 2006[1]). It's no secret that the Chinese government attempts to hack US firms[2].
Although I have no direct evidence that the Chinese government has installed backdoors/monitoring/etc. into Lenovo laptop hardware, I think there is much circumstantial evidence to warrant serious concerns. So much so that the U.S. government is restricting access to Lenovo computers[3].
I'm at a place with Lenovo laptops that I was with Kaspersky anti-virus[4]. Although it might arguably be easier to install nefarious stuff into software, why take chances with your hardware?
[1] http://news.bbc.co.uk/2/hi/americas/4997288.stm [2] http://www.businessinsider.com/china-hackers-us-companies-gi... [3] http://freebeacon.com/national-security/military-warns-chine... [4] https://www.usnews.com/news/business/articles/2017-09-14/con...
T5x0 - for a 15" Laptop
X2x0 - for 12" Laptops ( avoid the X2x0s)
W520, w530, P50, P51 - Desktop replacements 15" size
Insecure: http://www.makeuseof.com/tag/security-failings-demonstrate-a...
Some fry your lap: https://www.google.com/search?q=lenovo+hot
There is nothing wrong with used 3-4 years old machines. Processor power hasn't increased much. Just check the screen resolution, and factor in the extra cost if you need to add memory and swap the ssd disk.
The 440 series from 3 years ago didn't have physically separate mouse buttons on the touchpad (you just press down or tap at the corner areas of the touchpad), which some people found annoying.