2015 MBP 15" retina has the older, more reliable, keyboard, magsafe power, decent CPU and adequate GPU, HDMI, 2 mini-DP, regular USB 3 ports.
You could go older but there's no need unless you want a DVD drive. And you'd sacrifice CPU performance and maybe memory capacity.
Alas, it seems the Apple official Refurb store sold the last of their 2015 MBP models earlier this year. The earliest ones I see now (in US/UK/HK stores) are 2016 models.
You could swap out the DVD drives for a second hard drive. But that era MBP was a bit underpowered IMO. I had a great 2012 before the keyboard started failing. Now enjoy a 2015.
You are correct. But they're backward compatible with Mini DisplayPort. The only cables I've seen people plug in to them are minidp to {DVI, HDMI, DP}.
External hard drives are mostly USB3, and I don't know anyone with an external GPU.
The 2015 was the last of that body style, so it has the latest "guts" (CPU etc) with the previous keyboard and ports (USB-A, HDMI, Magsafe). Plus, with the non-mechanical touchpad, there's room for a slightly larger battery¹.
1: "The 2015 revision brought the modern Force Touch trackpad and used the space savings to increase the battery to 99.5 Wh, just under the 100 Wh carry-on limit for most commercial airlines" from Marco Arment, https://marco.org/2017/11/14/best-laptop-ever
You could go older but there's no need unless you want a DVD drive. And you'd sacrifice CPU performance and maybe memory capacity.
Alas, it seems the Apple official Refurb store sold the last of their 2015 MBP models earlier this year. The earliest ones I see now (in US/UK/HK stores) are 2016 models.