I'd like to hear more about the non-Apple options, but this is my research before buying a spare laptop last week.

This is the last year when you can buy the 2015 MacBook Pro with AppleCare warranty. Go to the Apple Refurbished store, and you should be able to find the old model. Refurbished means it was serviced by Apple during the warranty period. There's a maximum of 3 years warranty, therefore now is the last chance to buy the 2015 model. You can then add another 3 years AppleCare at checkout, and have Apple keep supporting it until 2021.

My main laptop is/was a 2014 Retina MacBook Pro, 512GB SSD, 16GB RAM. I accidentally booted into Diagnostics mode one day (holding down D on startup) and found a battery error PPT004. The warranty expires at the end of this week, so last Friday I took it to an AASP repair shop to try to get them to replace the battery before my warranty runs out.

While it goes away for 2 weeks, I decided to buy a spare laptop. I agonised over this choice for 2 days.

The MacBook Pro 2008 model can be bought for only $300 on eBay, and can take two 2.5" SSDs (one in the optical drive bay). The largest 2.5" SSD is 4 TB from Samsung. I really want that space. I could also recycle it as an external drive. The old keyboard with no gaps between the keys is really appealing. It can run macOS 10.12 (although I'd rather have 10.9). It has a replaceable battery (and I already have a spare), so I'd get 5 + 5 hours = 10 hours battery life.
However, I'd need to buy an ExpressCard adaptor to add USB 3 or SD reading. It also only has a maximum of 6 GB RAM. The screen is 15", but the resolution is only 1440x900. It's also heavy.

My other option was the MacBook Air 2013-2014 model. I specifically wanted those years, because they can run 10.9 (and even 10.8, so I can sync Notes without iCloud). I'd buy it locally, and get a Taiwanese keyboard with lots of special characters. It's also super light, and has SD and USB 3 built in. The screen is only 13" (not 15"), but has the same resolution as the 2008 model (1440x900). The battery can be replaced by unscrewing the case - it's not glued in like the Retina MacBook Pro. The battery life is 12 hours. The SSD is compatible with the Retina, so when I get my main laptop back from repair and buy a 2TB upgrade, the Air can take the hand-me-down 512GB. MagSafe 2 (which is great) is also going to be compatible between my 2013 Air and the 2014 Pro, unlike how it would be if I bought the 2008 Pro. The Air needs dongles for Ethernet and FireWire, but I have those for my Retina already.

If you want the MacBook Pro with an optical drive, only buy the 2012 model. Older models only have USB 2 and no Thunderbolt. The prices I found were much more expensive though, for lower battery life and comparable screen resolution.

In the end, I bought the 2013 MacBook Air. Downgrading the OS was hard! I had to mount in Thunderbolt Target Disk Mode, partition the drive, use DiskMaker X 5 to make one partition into a Mountain Lion installer, then reboot, install Mountain Lion on the other partition, backup with Time Machine, reboot into recovery mode, erase the drive again to make a single partition, and restore the Time Machine backup. It was hard, but it was possible, and I'm now typing this from the Air running 10.8. There's a few issues (Safari 6 doesn't let me see Github because of HTTPS issues, Skype doesn't run), and I might upgrade to 10.9 if those issues bother me too much.

If anyone has comments about running Linux on the MacBook Air - I'm also expecting to hop platforms soon, and I'll be using this spare laptop as a testing ground for that. Thankfully I resisted "upgrading" to a newer macOS that would change my drive to APFS. Linux should still be able to mount my HFS+ drive, because I stayed with the old OS. When my killer apps (iTunes & AppleScript) are ported, I'll be ready to jump, and start developing for Linux instead.