The newer Thinkpads has a Chiclet keyboard which are usually said to have an inferior layout to the older classic keyboard's 7 row layout, but it might have a better feel to type on than the classic keyboard, although this varies between people to people's preference.
Thinkpad when new are usually expensive, especially the X, T and W models. At least they're also long lasting and you'll usually find 5+ years old Thinkpad for cheap nowadays. These 2nd gen icore and even Core 2 Duo laptops are still capable these days, well as long as it's for general usage rather than more intensive tasks like 3D animation, CAD, or gaming, which it shouldn't be expected to be doing those anyway. However for just browsing sites, watching videos, shitposting, emulation, programming, or SSH/system administration, then they're absolutely fine.
Older models such as the X60, X61, T60, T61, and W500 are getting harder to find and usually the price might not be worth it (as of Ebay Q3 2017) so you should find a bit newer ones such as the X200, X201, X220, X230, T400, T410, T420, T430, W510, and W520.
If you can find the older ones for really cheap/off some thrift store or something, then go ahead and grab it while you can especially if you prefer the 4:3 screens.
SSD should always be beneficial over a HDD apart from its price-per-GiB. People may also say that it lasts less than an HDD, although an experiment found from 2014 that it lasted much longer than expected.
Still, data loss might be of a concern when it comes to having it without power for a long period of time, but really these days with modern SSD, it's going to be rather rare for it to happen.
In some Thinkpad models like the T420/X220, you can find a 9 cell battery which simply should have a longer battery life than the usual commonly supplied 6 cell battery.
Searching through Ebay, you should find some cheaper, generic battery made for Thinkpad, but are not from Lenovo but rather some no-name/Chinese company. You might be tempted to buy one, only because of the price. However you better be considering that:
At the end of the day, it's your pick. It's either the cheap but not as good battery or the expensive but reliable genuine Lenovo battery.
Usually the second hand/used Thinkpads comes with Windows 7 or 10. While it
might have better battery life than using Linux, it's still usually more
recourse intensive and it's preferred if you installed a Linux distribution
instead. Also Linux have great support for Thinkpads hardware anyway, you'll
find the TrackPoint to the FN keys working straight away.
Recommended Linux distros:
There's more but I can't be arse to list them all.
macOS/OSX/Hackintosh would be quite limited in support, but still possible
given you got the right model to use it. X220 should works with macOS Sierra.
Hackintosh Instructions, Hackintosh How to Guides